SQL - Constraints
Constraints are the rules enforced on data columns on table. These are used to limit the type of data that can go into a table. This ensures the accuracy and reliability of the data in the database.
Contraints could be column level or table level. Column level constraints are applied only to one column where as table level constraints are applied to the whole table.
Following are commonly used constraints available in SQL. These constraints have already been discussed in SQL - RDBMS Concepts chapter but its worth to revise them at this point.
- NOT NULL Constraint: Ensures that a column cannot have NULL value.
- DEFAULT Constraint : Provides a default value for a column when none is specified.
- UNIQUE Constraint: Ensures that all values in a column are different.
- PRIMARY Key: Uniquely identified each rows/records in a database table.
- FOREIGN Key: Uniquely identified a rows/records in any another database table.
- CHECK Constraint: The CHECK constraint ensures that all values in a column satisfy certain conditions.
- INDEX: Use to create and retrieve data from the database very quickly.
Constraints can be specified when a table is created with the CREATE TABLE statement or you can use ALTER TABLE statment to create constraints even after the table is created.
Dropping Constraints:
Any constraint that you have defined can be dropped using the ALTER TABLE command with the DROP CONSTRAINT option.
For example, to drop the primary key constraint in the EMPLOYEES table, you can use the following command:
ALTER TABLE EMPLOYEES DROP CONSTRAINT EMPLOYEES_PK;
Some implementations may provide shortcuts for dropping certain constraints. For example, to drop the primary key constraint for a table in Oracle, you can use the following command:
ALTER TABLE EMPLOYEES DROP PRIMARY KEY;
Some implementations allow you to disable constraints. Instead of permanently dropping a constraint from the database, you may want to temporarily disable the constraint, and then enable it later.
Integrity Constraints:
Integrity constraints are used to ensure accuracy and consistency of data in a relational database. Data integrity is handled in a relational database through the concept of referential integrity.
There are many types of integrity constraints that play a role in referential integrity (RI). These constraints include Primary Key, Foreign Key, Unique Constraints and other constraints mentioned above.
SQL - Using Joins
The SQL Joins clause is used to combine records from two or more tables in a database. A JOIN is a means for combining fields from two tables by using values common to each.
Consider following two tables, (a) CUSTOMERS table is as follows:
+----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 1 | Ramesh | 32 | Ahmedabad | 2000.00 | | 2 | Khilan | 25 | Delhi | 1500.00 | | 3 | kaushik | 23 | Kota | 2000.00 | | 4 | Chaitali | 25 | Mumbai | 6500.00 | | 5 | Hardik | 27 | Bhopal | 8500.00 | | 6 | Komal | 22 | MP | 4500.00 | | 7 | Muffy | 24 | Indore | 10000.00 | +----+----------+-----+-----------+----------+
(b) Another table is ORDERS as follows:
+-----+---------------------+-------------+--------+ |OID | DATE | CUSTOMER_ID | AMOUNT | +-----+---------------------+-------------+--------+ | 102 | 2009-10-08 00:00:00 | 3 | 3000 | | 100 | 2009-10-08 00:00:00 | 3 | 1500 | | 101 | 2009-11-20 00:00:00 | 2 | 1560 | | 103 | 2008-05-20 00:00:00 | 4 | 2060 | +-----+---------------------+-------------+--------+
Now let us join these two tables in our SELECT statement as follows:
SQL> SELECT ID, NAME, AGE, AMOUNT FROM CUSTOMERS, ORDERS WHERE CUSTOMERS.ID = ORDERS.CUSTOMER_ID;
This would produce following result:
+----+----------+-----+--------+ | ID | NAME | AGE | AMOUNT | +----+----------+-----+--------+ | 3 | kaushik | 23 | 3000 | | 3 | kaushik | 23 | 1500 | | 2 | Khilan | 25 | 1560 | | 4 | Chaitali | 25 | 2060 | +----+----------+-----+--------+
Here it is noteable that the join is performed in the WHERE clause. Several operators can be used to join tables, such as =, <, >, <>, <=, >=, !=, BETWEEN, LIKE, and NOT; they can all be used to join tables. However, the most common operator is the equal symbol.
SQL Join Types:
There are different type of joins available in SQL:
- INNER JOIN: returns rows when there is a match in both tables.
- LEFT JOIN: returns all rows from the left table, even if there are no matches in the right table.
- RIGHT JOIN: returns all rows from the right table, even if there are no matches in the left table.
- FULL JOIN: returns rows when there is a match in one of the tables.
- SELF JOIN: is used to join a table to itself, as if the table were two tables, temporarily renaming at least one table in the SQL statement.
CARTESIAN JOIN: returns the cartesian product of the sets of records from the two or more joined tables.
SQL - UNIONS CLAUSE
The SQL UNION clause/operator is used to combine the results of two or more SELECT statements without returning any duplicate rows.
To use UNION, each SELECT must have the same number of columns selected, the same number of column expressions, the same data type, and have them in the same order but they do not have to be the same length.
Syntax:
The basic syntax of UNION is as follows:
SELECT column1 [, column2 ] FROM table1 [, table2 ] [WHERE condition] UNION SELECT column1 [, column2 ] FROM table1 [, table2 ] [WHERE condition]
Here given condition could be any given expression based on your requirement.
Example:
Consider following two tables, (a) CUSTOMERS table is as follows:
+----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 1 | Ramesh | 32 | Ahmedabad | 2000.00 | | 2 | Khilan | 25 | Delhi | 1500.00 | | 3 | kaushik | 23 | Kota | 2000.00 | | 4 | Chaitali | 25 | Mumbai | 6500.00 | | 5 | Hardik | 27 | Bhopal | 8500.00 | | 6 | Komal | 22 | MP | 4500.00 | | 7 | Muffy | 24 | Indore | 10000.00 | +----+----------+-----+-----------+----------+
(b) Another table is ORDERS as follows:
+-----+---------------------+-------------+--------+ |OID | DATE | CUSTOMER_ID | AMOUNT | +-----+---------------------+-------------+--------+ | 102 | 2009-10-08 00:00:00 | 3 | 3000 | | 100 | 2009-10-08 00:00:00 | 3 | 1500 | | 101 | 2009-11-20 00:00:00 | 2 | 1560 | | 103 | 2008-05-20 00:00:00 | 4 | 2060 | +-----+---------------------+-------------+--------+
Now let us join these two tables in our SELECT statement as follows:
SQL> SELECT ID, NAME, AMOUNT, DATE FROM CUSTOMERS LEFT JOIN ORDERS ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID UNION SELECT ID, NAME, AMOUNT, DATE FROM CUSTOMERS RIGHT JOIN ORDERS ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;
This would produce following result:
+------+----------+--------+---------------------+ | ID | NAME | AMOUNT | DATE | +------+----------+--------+---------------------+ | 1 | Ramesh | NULL | NULL | | 2 | Khilan | 1560 | 2009-11-20 00:00:00 | | 3 | kaushik | 3000 | 2009-10-08 00:00:00 | | 3 | kaushik | 1500 | 2009-10-08 00:00:00 | | 4 | Chaitali | 2060 | 2008-05-20 00:00:00 | | 5 | Hardik | NULL | NULL | | 6 | Komal | NULL | NULL | | 7 | Muffy | NULL | NULL | +------+----------+--------+---------------------+
The UNION ALL Clause:
The UNION ALL operator is used to combine the results of two SELECT statements including duplicate rows.
The same rules that apply to UNION apply to the UNION ALL operator.
Syntax:
The basic syntax of UNION ALL is as follows:
SELECT column1 [, column2 ] FROM table1 [, table2 ] [WHERE condition] UNION ALL SELECT column1 [, column2 ] FROM table1 [, table2 ] [WHERE condition]
Here given condition could be any given expression based on your requirement.
Example:
Consider following two tables, (a) CUSTOMERS table is as follows:
+----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 1 | Ramesh | 32 | Ahmedabad | 2000.00 | | 2 | Khilan | 25 | Delhi | 1500.00 | | 3 | kaushik | 23 | Kota | 2000.00 | | 4 | Chaitali | 25 | Mumbai | 6500.00 | | 5 | Hardik | 27 | Bhopal | 8500.00 | | 6 | Komal | 22 | MP | 4500.00 | | 7 | Muffy | 24 | Indore | 10000.00 | +----+----------+-----+-----------+----------+
(b) Another table is ORDERS as follows:
+-----+---------------------+-------------+--------+ |OID | DATE | CUSTOMER_ID | AMOUNT | +-----+---------------------+-------------+--------+ | 102 | 2009-10-08 00:00:00 | 3 | 3000 | | 100 | 2009-10-08 00:00:00 | 3 | 1500 | | 101 | 2009-11-20 00:00:00 | 2 | 1560 | | 103 | 2008-05-20 00:00:00 | 4 | 2060 | +-----+---------------------+-------------+--------+
Now let us join these two tables in our SELECT statement as follows:
SQL> SELECT ID, NAME, AMOUNT, DATE FROM CUSTOMERS LEFT JOIN ORDERS ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID UNION ALL SELECT ID, NAME, AMOUNT, DATE FROM CUSTOMERS RIGHT JOIN ORDERS ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;
This would produce following result:
+------+----------+--------+---------------------+ | ID | NAME | AMOUNT | DATE | +------+----------+--------+---------------------+ | 1 | Ramesh | NULL | NULL | | 2 | Khilan | 1560 | 2009-11-20 00:00:00 | | 3 | kaushik | 3000 | 2009-10-08 00:00:00 | | 3 | kaushik | 1500 | 2009-10-08 00:00:00 | | 4 | Chaitali | 2060 | 2008-05-20 00:00:00 | | 5 | Hardik | NULL | NULL | | 6 | Komal | NULL | NULL | | 7 | Muffy | NULL | NULL | | 3 | kaushik | 3000 | 2009-10-08 00:00:00 | | 3 | kaushik | 1500 | 2009-10-08 00:00:00 | | 2 | Khilan | 1560 | 2009-11-20 00:00:00 | | 4 | Chaitali | 2060 | 2008-05-20 00:00:00 | +------+----------+--------+---------------------+
There are two other clauses (i.e operators ) which are very similar to UNION clause:
- SQL INTERSECT Clause: is used to combine two SELECT statements, but returns rows only from the first SELECT statement that are identical to a row in the second SELECT statement.
SQL EXCEPT Clause : combines two SELECT statements and returns rows from the first SELECT statement that are not returned by the second SELECT statement.
SQL - NULL Values
The SQL NULL is the term used to represent a missing value. A NULL value in a table is a value in a field that appears to be blank.
A field with a NULL value is a field with no value. It is very important to understand that a NULL value is different than a zero value or a field that contains spaces.
Syntax:
The basic syntax of NULL while creating a table:
SQL> CREATE TABLE CUSTOMERS( ID INT NOT NULL, NAME VARCHAR (20) NOT NULL, AGE INT NOT NULL, ADDRESS CHAR (25) , SALARY DECIMAL (18, 2), PRIMARY KEY (ID) );
Here NOT NULL signifies that column should always accept an explicit value of the given data type. There are two column where we did not use NOT NULL which means these column could be NULL.
A field with a NULL value is one that has been left blank during record creation.
Example:
The NULL value can cause problems when selecting data, however, because when comparing an unknown value to any other value, the result is always unknown and not included in the final results.
You must use the IS NULL or IS NOT NULL operators in order to check for a NULL value.
Consider following table, CUSTOMERS having following records:
+----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 1 | Ramesh | 32 | Ahmedabad | 2000.00 | | 2 | Khilan | 25 | Delhi | 1500.00 | | 3 | kaushik | 23 | Kota | 2000.00 | | 4 | Chaitali | 25 | Mumbai | 6500.00 | | 5 | Hardik | 27 | Bhopal | 8500.00 | | 6 | Komal | 22 | MP | | | 7 | Muffy | 24 | Indore | | +----+----------+-----+-----------+----------+
Now following is the usage of IS NOT NULL operator:
SQL> SELECT ID, NAME, AGE, ADDRESS, SALARY FROM CUSTOMERS WHERE SALARY IS NOT NULL;
This would produce following result:
+----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 1 | Ramesh | 32 | Ahmedabad | 2000.00 | | 2 | Khilan | 25 | Delhi | 1500.00 | | 3 | kaushik | 23 | Kota | 2000.00 | | 4 | Chaitali | 25 | Mumbai | 6500.00 | | 5 | Hardik | 27 | Bhopal | 8500.00 | +----+----------+-----+-----------+----------+
Now following is the usage of IS NULL operator:
SQL> SELECT ID, NAME, AGE, ADDRESS, SALARY FROM CUSTOMERS WHERE SALARY IS NULL;
This would produce following result:
+----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 6 | Komal | 22 | MP | | | 7 | Muffy | 24 | Indore | | +----+----------+-----+-----------+----------+
SQL - Alias Syntax
You can rename a table or a column temporarily by giving another name known as alias.
The use of table aliases means to rename a table in a particular SQL statement. The renaming is a temporary change and the actual table name does not change in the database.
The column aliases are used to rename a table's columns for the purpose of a particular SQL query.
Syntax:
The basic syntax of table alias is as follows:
SELECT column1, column2.... FROM table_name AS alias_name WHERE [condition];
The basic syntax of column alias is as follows:
SELECT column_name AS alias_name FROM table_name WHERE [condition];
Example:
Consider following two tables, (a) CUSTOMERS table is as follows:
+----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 1 | Ramesh | 32 | Ahmedabad | 2000.00 | | 2 | Khilan | 25 | Delhi | 1500.00 | | 3 | kaushik | 23 | Kota | 2000.00 | | 4 | Chaitali | 25 | Mumbai | 6500.00 | | 5 | Hardik | 27 | Bhopal | 8500.00 | | 6 | Komal | 22 | MP | 4500.00 | | 7 | Muffy | 24 | Indore | 10000.00 | +----+----------+-----+-----------+----------+
(b) Another table is ORDERS as follows:
+-----+---------------------+-------------+--------+ |OID | DATE | CUSTOMER_ID | AMOUNT | +-----+---------------------+-------------+--------+ | 102 | 2009-10-08 00:00:00 | 3 | 3000 | | 100 | 2009-10-08 00:00:00 | 3 | 1500 | | 101 | 2009-11-20 00:00:00 | 2 | 1560 | | 103 | 2008-05-20 00:00:00 | 4 | 2060 | +-----+---------------------+-------------+--------+
Now following is the usage of table alias:
SQL> SELECT C.ID, C.NAME, C.AGE, O.AMOUNT FROM CUSTOMERS AS C, ORDERS AS O WHERE C.ID = O.CUSTOMER_ID;
This would produce following result:
+----+----------+-----+--------+ | ID | NAME | AGE | AMOUNT | +----+----------+-----+--------+ | 3 | kaushik | 23 | 3000 | | 3 | kaushik | 23 | 1500 | | 2 | Khilan | 25 | 1560 | | 4 | Chaitali | 25 | 2060 | +----+----------+-----+--------+
Following is the usage of column alias:
SQL> SELECT ID AS CUSTOMER_ID, NAME AS CUSTOMER_NAME FROM CUSTOMERS WHERE SALARY IS NOT NULL;
This would produce following result:
+-------------+---------------+ | CUSTOMER_ID | CUSTOMER_NAME | +-------------+---------------+ | 1 | Ramesh | | 2 | Khilan | | 3 | kaushik | | 4 | Chaitali | | 5 | Hardik | | 6 | Komal | | 7 | Muffy | +-------------+---------------+
SQL - Indexes
Indexes are special lookup tables that the database search engine can use to speed up data retrieval. Simply put, an index is a pointer to data in a table. An index in a database is very similar to an index in the back of a book.
For example, if you want to reference all pages in a book that discuss a certain topic, you first refer to the index, which lists all topics alphabetically, and are then referred to one or more specific page numbers.
An index helps speed up SELECT queries and WHERE clauses, but it slows down data input, with UPDATE and INSERT statements. Indexes can be created or dropped with no effect on the data.
Creating an index involves the CREATE INDEX statement, which allows you to name the index, to specify the table and which column or columns to index, and to indicate whether the index is in ascending or descending order.
Indexes can also be unique, similar to the UNIQUE constraint, in that the index prevents duplicate entries in the column or combination of columns on which there's an index.
The CREATE INDEX Command:
The basic syntax of CREATE INDEX is as follows:
CREATE INDEX index_name ON table_name;
Single-Column Indexes:
A single-column index is one that is created based on only one table column. The basic syntax is as follows:
CREATE INDEX index_name ON table_name (column_name);
Unique Indexes:
Unique indexes are used not only for performance, but also for data integrity. A unique index does not allow any duplicate values to be inserted into the table. The basic syntax is as follows:
CREATE INDEX index_name on table_name (column_name);
Composite Indexes:
A composite index is an index on two or more columns of a table. The basic syntax is as follows:
CREATE INDEX index_name on table_name (column1, column2);
Whether to create a single-column index or a composite index, take into consideration the column(s) that you may use very frequently in a query's WHERE clause as filter conditions.
Should there be only one column used, a single-column index should be the choice. Should there be two or more columns that are frequently used in the WHERE clause as filters, the composite index would be the best choice.
Implicit Indexes:
Implicit indexes are indexes that are automatically created by the database server when an object is created. Indexes are automatically created for primary key constraints and unique constraints.
The DROP INDEX Command:
An index can be dropped using SQL DROP command. Care should be taken when dropping an index because performance may be slowed or improved.
The basic syntax is as follows:
DROP INDEX index_name;
When should indexes be avoided?
Although indexes are intended to enhance a database's performance, there are times when they should be avoided. The following guidelines indicate when the use of an index should be reconsidered:
- Indexes should not be used on small tables.
- Tables that have frequent, large batch update or insert operations.
- Indexes should not be used on columns that contain a high number of NULL values.
- Columns that are frequently manipulated should not be indexed.
SQL - ALTER TABLE Command
The SQL ALTER TABLE command is used to add, delete, or modify columns in an existing table.
You would also use ALTER TABLE command to add and drop various constraints on a an existing table.
Syntax:
The basic syntax of ALTER TABLE to add a new column in an existing table is as follows:
ALTER TABLE table_name ADD column_name datatype;
The basic syntax of ALTER TABLE to DROP COLUMN in an existing table is as follows:
ALTER TABLE table_name DROP COLUMN column_name;
The basic syntax of ALTER TABLE to change the DATA TYPE of a column in a table is as follows:
ALTER TABLE table_name MODIFY COLUMN column_name datatype;
The basic syntax of ALTER TABLE to add a NOT NULL constraint to a column in a table is as follows:
ALTER TABLE table_name MODIFY column_name datatype NOT NULL;
The basic syntax of ALTER TABLE to ADD UNIQUE CONSTRAINT to a table is as follows:
ALTER TABLE table_name ADD CONSTRAINT MyUniqueConstraint UNIQUE(column1, column2...);
The basic syntax of ALTER TABLE to ADD CHECK CONSTRAINT to a table is as follows:
ALTER TABLE table_name ADD CONSTRAINT MyUniqueConstraint CHECK (CONDITION);
The basic syntax of ALTER TABLE to ADD PRIMARY KEY constraint to a table is as follows:
ALTER TABLE table_name ADD CONSTRAINT MyPrimaryKey PRIMARY KEY (column1, column2...);
The basic syntax of ALTER TABLE to DROP CONSTRAINT from a table is as follows:
ALTER TABLE table_name DROP CONSTRAINT MyUniqueConstraint;
If you're using MySQL, the code is as follows:
ALTER TABLE table_name DROP INDEX MyUniqueConstraint;
The basic syntax of ALTER TABLE to DROP PRIMARY KEY constraint from a table is as follows:
ALTER TABLE table_name DROP CONSTRAINT MyPrimaryKey;
If you're using MySQL, the code is as follows:
ALTER TABLE table_name DROP PRIMARY KEY;
Example:
Consider CUSTOMERS table is having following records:
+----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 1 | Ramesh | 32 | Ahmedabad | 2000.00 | | 2 | Khilan | 25 | Delhi | 1500.00 | | 3 | kaushik | 23 | Kota | 2000.00 | | 4 | Chaitali | 25 | Mumbai | 6500.00 | | 5 | Hardik | 27 | Bhopal | 8500.00 | | 6 | Komal | 22 | MP | 4500.00 | | 7 | Muffy | 24 | Indore | 10000.00 | +----+----------+-----+-----------+----------+
Following is the example to ADD a new column in an existing table:
ALTER TABLE CUSTOMERS ADD SEX char(1);
Now CUSTOMERS table is changed and following would be output from SELECT statement:
+----+---------+-----+-----------+----------+------+ | ID | NAME | AGE | ADDRESS | SALARY | SEX | +----+---------+-----+-----------+----------+------+ | 1 | Ramesh | 32 | Ahmedabad | 2000.00 | NULL | | 2 | Ramesh | 25 | Delhi | 1500.00 | NULL | | 3 | kaushik | 23 | Kota | 2000.00 | NULL | | 4 | kaushik | 25 | Mumbai | 6500.00 | NULL | | 5 | Hardik | 27 | Bhopal | 8500.00 | NULL | | 6 | Komal | 22 | MP | 4500.00 | NULL | | 7 | Muffy | 24 | Indore | 10000.00 | NULL | +----+---------+-----+-----------+----------+------+
Following is the example to DROP sex column from existing table:
ALTER TABLE CUSTOMERS DROP SEX;
Now CUSTOMERS table is changed and following would be output from SELECT statement:
+----+---------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+---------+-----+-----------+----------+ | 1 | Ramesh | 32 | Ahmedabad | 2000.00 | | 2 | Ramesh | 25 | Delhi | 1500.00 | | 3 | kaushik | 23 | Kota | 2000.00 | | 4 | kaushik | 25 | Mumbai | 6500.00 | | 5 | Hardik | 27 | Bhopal | 8500.00 | | 6 | Komal | 22 | MP | 4500.00 | | 7 | Muffy | 24 | Indore | 10000.00 | +----+---------+-----+-----------+----------+
SQL - TRUNCATE TABLE Command
The SQL TRUNCATE TABLE command is used to delete complete data from an existing table.
You can also use DROP TABLE command to delete complete table but it would remove complete table structure form the database and you would need to re-create this table once again if you wish you store some data.
Syntax:
The basic syntax of TRUNCATE TABLE is as follows:
TRUNCATE TABLE table_name;
Example:
Consider CUSTOMERS table is having following records:
+----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 1 | Ramesh | 32 | Ahmedabad | 2000.00 | | 2 | Khilan | 25 | Delhi | 1500.00 | | 3 | kaushik | 23 | Kota | 2000.00 | | 4 | Chaitali | 25 | Mumbai | 6500.00 | | 5 | Hardik | 27 | Bhopal | 8500.00 | | 6 | Komal | 22 | MP | 4500.00 | | 7 | Muffy | 24 | Indore | 10000.00 | +----+----------+-----+-----------+----------+
Following is the example to turncate:
SQL > TRUNCATE TABLE CUSTOMERS;
Now CUSTOMERS table is truncated and following would be output from SELECT statement:
SQL> SELECT * FROM CUSTOMERS; Empty set (0.00 sec)
SQL - Using Views
A view is nothing more than a SQL statement that is stored in the database with an associated name. A view is actually a composition of a table in the form of a predefined SQL query.
A view can contain all rows of a table or select rows from a table. A view can be created from one or many tables which depends on the written SQL query to create a view.
Views which are kind of virtual tables, allow users to do the following:
- Structure data in a way that users or classes of users find natural or intuitive.
- Restrict access to the data such that a user can see and (sometimes) modify exactly what they need and no more.
- Summarize data from various tables which can be used to generate reports.
Creating Views:
Database views are created using the CREATE VIEW statement. Views can be created from a single table, multiple tables, or another view.
To create a view, a user must have the appropriate system privilege according to the specific implementation.
The basic CREATE VIEW syntax is as follows:
CREATE VIEW view_name AS SELECT column1, column2..... FROM table_name WHERE [condition];
You can include multiple tables in your SELECT statement in very similar way as you use them in normal SQL SELECT query.
Example:
Consider CUSTOMERS table is having following records:
+----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 1 | Ramesh | 32 | Ahmedabad | 2000.00 | | 2 | Khilan | 25 | Delhi | 1500.00 | | 3 | kaushik | 23 | Kota | 2000.00 | | 4 | Chaitali | 25 | Mumbai | 6500.00 | | 5 | Hardik | 27 | Bhopal | 8500.00 | | 6 | Komal | 22 | MP | 4500.00 | | 7 | Muffy | 24 | Indore | 10000.00 | +----+----------+-----+-----------+----------+
Now, following is the example to create a view from CUSTOMERS table. This view would be used to have customer name and age from CUSTOMERS table:
SQL > CREATE VIEW CUSTOMERS_VIEW AS SELECT name, age FROM CUSTOMERS;
Now you can query CUSTOMERS_VIEW in similar way as you query an actual table. Following is the example:
SQL > SELECT * FROM CUSTOMERS_VIEW;
This would produce following result:
+----------+-----+ | name | age | +----------+-----+ | Ramesh | 32 | | Khilan | 25 | | kaushik | 23 | | Chaitali | 25 | | Hardik | 27 | | Komal | 22 | | Muffy | 24 | +----------+-----+
The WITH CHECK OPTION:
The WITH CHECK OPTION is a CREATE VIEW statement option. The purpose of the WITH CHECK OPTION is to ensure that all UPDATE and INSERTs satisfy the condition(s) in the view definition.
If they do not satisfy the condition(s), the UPDATE or INSERT returns an error.
The following is an example of creating same view CUSTOMERS_VIEW with the WITH CHECK OPTION:
CREATE VIEW CUSTOMERS_VIEW AS SELECT name, age FROM CUSTOMERS WHERE age IS NOT NULL WITH CHECK OPTION;
The WITH CHECK OPTION in this case should deny the entry of any NULL values in the view's AGE column, because the view is defined by data that does not have a NULL value in the AGE column.
Updating a View:
A view can be updated under certain conditions:
- The SELECT clause may not contain the keyword DISTINCT.
- The SELECT clause may not contain summary functions.
- The SELECT clause may not contain set functions.
- The SELECT clause may not contain set operators.
- The SELECT clause may not contain an ORDER BY clause.
- The FROM clause may not contain multiple tables.
- The WHERE clause may not contain subqueries.
- The query may not contain GROUP BY or HAVING.
- Calculated columns may not be updated.
- All NOT NULL columns from the base table must be included in the view in order for the INSERT query to function.
So if a view satisties all the above mentioned rules then you can update a view. Following is an example to update the age of Ramesh:
SQL > UPDATE CUSTOMERS_VIEW SET AGE = 35 WHERE name='Ramesh';
This would ultimately update the base table CUSTOMERS and same would reflect in the view itself. Now try to query base table, and SELECT statement would produce following result:
+----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 1 | Ramesh | 35 | Ahmedabad | 2000.00 | | 2 | Khilan | 25 | Delhi | 1500.00 | | 3 | kaushik | 23 | Kota | 2000.00 | | 4 | Chaitali | 25 | Mumbai | 6500.00 | | 5 | Hardik | 27 | Bhopal | 8500.00 | | 6 | Komal | 22 | MP | 4500.00 | | 7 | Muffy | 24 | Indore | 10000.00 | +----+----------+-----+-----------+----------+
Inserting Rows into a View:
Rows of data can be inserted into a view. The same rules that apply to the UPDATE command also apply to the INSERT command.
Here we can not insert rows in CUSTOMERS_VIEW because we have not included all the NOT NULL columns in this view, otherwise you can insert rows in a view in similar way as you insert them in a table.
Deleting Rows into a View:
Rows of data can be deleted from a view. The same rules that apply to the UPDATE and INSERT commands apply to the DELETE command.
Following is an example to delete a record having AGE= 22.
SQL > DELETE FROM CUSTOMERS_VIEW WHERE age = 22;
This would ultimately delete a row from the base table CUSTOMERS and same would reflect in the view itself. Now try to query base table, and SELECT statement would produce following result:
+----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 1 | Ramesh | 35 | Ahmedabad | 2000.00 | | 2 | Khilan | 25 | Delhi | 1500.00 | | 3 | kaushik | 23 | Kota | 2000.00 | | 4 | Chaitali | 25 | Mumbai | 6500.00 | | 5 | Hardik | 27 | Bhopal | 8500.00 | | 7 | Muffy | 24 | Indore | 10000.00 | +----+----------+-----+-----------+----------+
Dropping Views:
Obviously, where you have a view, you need a way to drop the view if it is no longer needed. The syntax is very simple as given below:
DROP VIEW view_name;
Following is an example to drop CUSTOMERS_VIEW from CUSTOMERS table:
DROP VIEW CUSTOMERS_VIEW;
SQL - Having Clause
The HAVING clause enables you to specify conditions that filter which group results appear in the final results.
The WHERE clause places conditions on the selected columns, whereas the HAVING clause places conditions on groups created by the GROUP BY clause.
Syntax:
The following is the position of the HAVING clause in a query:
SELECT
FROM
WHERE
GROUP BY
HAVING
ORDER BY
The HAVING clause must follow the GROUP BY clause in a query and must also precede the ORDER BY clause if used. The following is the syntax of the SELECT statement, including the HAVING clause:
SELECT column1, column2 FROM table1, table2 WHERE [ conditions ] GROUP BY column1, column2 HAVING [ conditions ] ORDER BY column1, column2
Example:
Consider CUSTOMERS table is having following records:
+----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 1 | Ramesh | 32 | Ahmedabad | 2000.00 | | 2 | Khilan | 25 | Delhi | 1500.00 | | 3 | kaushik | 23 | Kota | 2000.00 | | 4 | Chaitali | 25 | Mumbai | 6500.00 | | 5 | Hardik | 27 | Bhopal | 8500.00 | | 6 | Komal | 22 | MP | 4500.00 | | 7 | Muffy | 24 | Indore | 10000.00 | +----+----------+-----+-----------+----------+
Following is the example which would display record for which similar age count would be more than or equal to 2:
SQL > SELECT * FROM CUSTOMERS GROUP BY age HAVING COUNT(age) >= 2;
This would produce following result:
+----+--------+-----+---------+---------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+--------+-----+---------+---------+ | 2 | Khilan | 25 | Delhi | 1500.00 | +----+--------+-----+---------+---------+
SQL - Transactions
A transaction is a unit of work that is performed against a database. Transactions are units or sequences of work accomplished in a logical order, whether in a manual fashion by a user or automatically by some sort of a database program.
A transaction is the propagation of one or more changes to the database. For example, if you are creating a record or updating a record or deleting a record from the table then you are performing transaction on the table. It is important to control transactions to ensure data integrity and to handle database errors.
Practically you will club many SQL queries into a group and you will execute all of them together as a part of a transaction.
Properties of Transactions:
Transactions have the following four standard properties, usually referred to by the acronym ACID:
- Atomicity: ensures that all operations within the work unit are completed successfully; otherwise, the transaction is aborted at the point of failure, and previous operations are rolled back to their former state.
- Consistency: ensures that the database properly changes states upon a successfully committed transaction.
- Isolation: enables transactions to operate independently of and transparent to each other.
- Durability: ensures that the result or effect of a committed transaction persists in case of a system failure.
Transaction Control:
There are following commands used to control transactions:
- COMMIT: to save the changes.
- ROLLBACK: to rollback the changes.
- SAVEPOINT: creates points within groups of transactions in which to ROLLBACK
- SET TRANSACTION: Places a name on a transaction.
Transactional control commands are only used with the DML commands INSERT, UPDATE, and DELETE only. They can not be used while creating tables or dropping them because these operations are automatically commited in the database.
The COMMIT Command:
The COMMIT command is the transactional command used to save changes invoked by a transaction to the database.
The COMMIT command saves all transactions to the database since the last COMMIT or ROLLBACK command.
The syntax for COMMIT command is as follows:
COMMIT;
Example:
Consider CUSTOMERS table is having following records:
+----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 1 | Ramesh | 32 | Ahmedabad | 2000.00 | | 2 | Khilan | 25 | Delhi | 1500.00 | | 3 | kaushik | 23 | Kota | 2000.00 | | 4 | Chaitali | 25 | Mumbai | 6500.00 | | 5 | Hardik | 27 | Bhopal | 8500.00 | | 6 | Komal | 22 | MP | 4500.00 | | 7 | Muffy | 24 | Indore | 10000.00 | +----+----------+-----+-----------+----------+
Following is the example which would delete records from the table having age = 25, and then COMMIT the changes in the database.
SQL> DELETE FROM CUSTOMERS WHERE AGE = 25; SQL> COMMIT;
As a result, two rows from the table would be deleted and SELECT statement would produce following result:
+----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 1 | Ramesh | 32 | Ahmedabad | 2000.00 | | 3 | kaushik | 23 | Kota | 2000.00 | | 5 | Hardik | 27 | Bhopal | 8500.00 | | 6 | Komal | 22 | MP | 4500.00 | | 7 | Muffy | 24 | Indore | 10000.00 | +----+----------+-----+-----------+----------+
The ROLLBACK Command:
The ROLLBACK command is the transactional command used to undo transactions that have not already been saved to the database.
The ROLLBACK command can only be used to undo transactions since the last COMMIT or ROLLBACK command was issued.
The syntax for ROLLBACK command is as follows:
ROLLBACK;
Example:
Consider CUSTOMERS table is having following records:
+----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 1 | Ramesh | 32 | Ahmedabad | 2000.00 | | 2 | Khilan | 25 | Delhi | 1500.00 | | 3 | kaushik | 23 | Kota | 2000.00 | | 4 | Chaitali | 25 | Mumbai | 6500.00 | | 5 | Hardik | 27 | Bhopal | 8500.00 | | 6 | Komal | 22 | MP | 4500.00 | | 7 | Muffy | 24 | Indore | 10000.00 | +----+----------+-----+-----------+----------+
Following is the example which would delete records from the table having age = 25, and then ROLLBACK the changes in the database.
SQL> DELETE FROM CUSTOMERS WHERE AGE = 25; SQL> ROLLBACK;
As a result, delete operation would not impact the table and SELECT statement would produce following result:
+----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 1 | Ramesh | 32 | Ahmedabad | 2000.00 | | 2 | Khilan | 25 | Delhi | 1500.00 | | 3 | kaushik | 23 | Kota | 2000.00 | | 4 | Chaitali | 25 | Mumbai | 6500.00 | | 5 | Hardik | 27 | Bhopal | 8500.00 | | 6 | Komal | 22 | MP | 4500.00 | | 7 | Muffy | 24 | Indore | 10000.00 | +----+----------+-----+-----------+----------+
The SAVEPOINT Command:
A SAVEPOINT is a point in a transaction when you can roll the transaction back to a certain point without rolling back the entire transaction.
The syntax for SAVEPOINT command is as follows:
SAVEPOINT SAVEPOINT_NAME;
This command serves only in the creation of a SAVEPOINT among transactional statements. The ROLLBACK command is used to undo a group of transactions.
The syntax for rolling back to a SAVEPOINT is as follows:
ROLLBACK TO SAVEPOINT_NAME;
Following is an example where you plan to delete the three different records from the CUSTOMERS table. You want to create a SAVEPOINT before each delete, so that you can ROLLBACK to any SAVEPOINT at any time to return the appropriate data to its original state:
Example:
Consider CUSTOMERS table is having following records:
+----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 1 | Ramesh | 32 | Ahmedabad | 2000.00 | | 2 | Khilan | 25 | Delhi | 1500.00 | | 3 | kaushik | 23 | Kota | 2000.00 | | 4 | Chaitali | 25 | Mumbai | 6500.00 | | 5 | Hardik | 27 | Bhopal | 8500.00 | | 6 | Komal | 22 | MP | 4500.00 | | 7 | Muffy | 24 | Indore | 10000.00 | +----+----------+-----+-----------+----------+
Now here is the series of operations:
SQL> SAVEPOINT SP1; Savepoint created. SQL> DELETE FROM CUSTOMERS WHERE ID=1; 1 row deleted. SQL> SAVEPOINT SP2; Savepoint created. SQL> DELETE FROM CUSTOMERS WHERE ID=2; 1 row deleted. SQL> SAVEPOINT SP3; Savepoint created. SQL> DELETE FROM CUSTOMERS WHERE ID=3; 1 row deleted.
Now that the three deletions have taken place, say you have changed your mind and decided to ROLLBACK to the SAVEPOINT that you identified as SP2. Because SP2 was created after the first deletion, the last two deletions are undone:
SQL> ROLLBACK TO SP2; Rollback complete.
Notice that only the first deletion took place since you rolled back to SP2:
SQL> SELECT * FROM CUSTOMERS; +----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 2 | Khilan | 25 | Delhi | 1500.00 | | 3 | kaushik | 23 | Kota | 2000.00 | | 4 | Chaitali | 25 | Mumbai | 6500.00 | | 5 | Hardik | 27 | Bhopal | 8500.00 | | 6 | Komal | 22 | MP | 4500.00 | | 7 | Muffy | 24 | Indore | 10000.00 | +----+----------+-----+-----------+----------+ 6 rows selected.
The RELEASE SAVEPOINT Command:
The RELEASE SAVEPOINT command is used to remove a SAVEPOINT that you have created.
The syntax for RELEASE SAVEPOINT is as follows:
RELEASE SAVEPOINT SAVEPOINT_NAME;
Once a SAVEPOINT has been released, you can no longer use the ROLLBACK command to undo transactions performed since the SAVEPOINT.
The SET TRANSACTION Command:
The SET TRANSACTION command can be used to initiate a database transaction. This command is used to specify characteristics for the transaction that follows.
For example, you can specify a transaction to be read only, or read write.
The syntax for SET TRANSACTION is as follows:
SET TRANSACTION [ READ WRITE | READ ONLY ];
SQL - Wildcard Operators
We already have discussed SQL LIKE operator which is used to compare a value to similar values using wildcard operators.
SQL supports following two wildcard operators in conjunction with the LIKE operator:
| Wildcards | Description |
|---|---|
| The percent sign (%) | Matches one or more characters. Note that MS Access uses the asterisk (*) wildcard character instead of the percent sign (%) wildcard character. |
| The underscore (_) | Matches one character. Note that MS Access uses a question mark (?) instead of the underscore (_) to match any one character. |
The percent sign represents zero, one, or multiple characters. The underscore represents a single number or character. The symbols can be used in combinations.
Syntax:
The basic syntax of % and _ is as follows:
SELECT FROM table_name WHERE column LIKE 'XXXX%' or SELECT FROM table_name WHERE column LIKE '%XXXX%' or SELECT FROM table_name WHERE column LIKE 'XXXX_' or SELECT FROM table_name WHERE column LIKE '_XXXX' or SELECT FROM table_name WHERE column LIKE '_XXXX_'
You can combine N number of conditions using AND or OR operators. Here XXXX could be any numberic or string value.
Example:
Here are number of examples showing WHERE part having different LIKE clause with '%' and '_' operators:
| Statement | Description |
|---|---|
| WHERE SALARY LIKE '200%' | Finds any values that start with 200 |
| WHERE SALARY LIKE '%200%' | Finds any values that have 200 in any position |
| WHERE SALARY LIKE '_00%' | Finds any values that have 00 in the second and third positions |
| WHERE SALARY LIKE '2_%_%' | Finds any values that start with 2 and are at least 3 characters in length |
| WHERE SALARY LIKE '%2' | Finds any values that end with 2 |
| WHERE SALARY LIKE '_2%3' | Finds any values that have a 2 in the second position and end with a 3 |
| WHERE SALARY LIKE '2___3' | Finds any values in a five-digit number that start with 2 and end with 3 |
Let us take a real example, consider CUSTOMERS table is having following records:
+----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 1 | Ramesh | 32 | Ahmedabad | 2000.00 | | 2 | Khilan | 25 | Delhi | 1500.00 | | 3 | kaushik | 23 | Kota | 2000.00 | | 4 | Chaitali | 25 | Mumbai | 6500.00 | | 5 | Hardik | 27 | Bhopal | 8500.00 | | 6 | Komal | 22 | MP | 4500.00 | | 7 | Muffy | 24 | Indore | 10000.00 | +----+----------+-----+-----------+----------+
Following is an example which would display all the records from CUSTOMERS table where SALARY starts with 200:
SQL> SELECT * FROM CUSTOMERS WHERE SALARY LIKE '200%';
This would produce following result:
+----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 1 | Ramesh | 32 | Ahmedabad | 2000.00 | | 3 | kaushik | 23 | Kota | 2000.00 | +----+----------+-----+-----------+----------+
SQL - Date Functions
Following is the list of all important Date and Time related functions available through SQL. There are various other functions supported by your RDBMS. Given list is based on MySQL RDBMS.
| Name | Description |
|---|---|
| ADDDATE() | Add dates |
| ADDTIME() | Add time |
| CONVERT_TZ() | Convert from one timezone to another |
| CURDATE() | Return the current date |
| CURRENT_DATE(), CURRENT_DATE | Synonyms for CURDATE() |
| CURRENT_TIME(), CURRENT_TIME | Synonyms for CURTIME() |
| CURRENT_TIMESTAMP(), CURRENT_TIMESTAMP | Synonyms for NOW() |
| CURTIME() | Return the current time |
| DATE_ADD() | Add two dates |
| DATE_FORMAT() | Format date as specified |
| DATE_SUB() | Subtract two dates |
| DATE() | Extract the date part of a date or datetime expression |
| DATEDIFF() | Subtract two dates |
| DAY() | Synonym for DAYOFMONTH() |
| DAYNAME() | Return the name of the weekday |
| DAYOFMONTH() | Return the day of the month (1-31) |
| DAYOFWEEK() | Return the weekday index of the argument |
| DAYOFYEAR() | Return the day of the year (1-366) |
| EXTRACT | Extract part of a date |
| FROM_DAYS() | Convert a day number to a date |
| FROM_UNIXTIME() | Format date as a UNIX timestamp |
| HOUR() | Extract the hour |
| LAST_DAY | Return the last day of the month for the argument |
| LOCALTIME(), LOCALTIME | Synonym for NOW() |
| LOCALTIMESTAMP, LOCALTIMESTAMP() | Synonym for NOW() |
| MAKEDATE() | Create a date from the year and day of year |
| MAKETIME | MAKETIME() |
| MICROSECOND() | Return the microseconds from argument |
| MINUTE() | Return the minute from the argument |
| MONTH() | Return the month from the date passed |
| MONTHNAME() | Return the name of the month |
| NOW() | Return the current date and time |
| PERIOD_ADD() | Add a period to a year-month |
| PERIOD_DIFF() | Return the number of months between periods |
| QUARTER() | Return the quarter from a date argument |
| SEC_TO_TIME() | Converts seconds to 'HH:MM:SS' format |
| SECOND() | Return the second (0-59) |
| STR_TO_DATE() | Convert a string to a date |
| SUBDATE() | When invoked with three arguments a synonym for DATE_SUB() |
| SUBTIME() | Subtract times |
| SYSDATE() | Return the time at which the function executes |
| TIME_FORMAT() | Format as time |
| TIME_TO_SEC() | Return the argument converted to seconds |
| TIME() | Extract the time portion of the expression passed |
| TIMEDIFF() | Subtract time |
| TIMESTAMP() | With a single argument, this function returns the date or datetime expression. With two arguments, the sum of the arguments |
| TIMESTAMPADD() | Add an interval to a datetime expression |
| TIMESTAMPDIFF() | Subtract an interval from a datetime expression |
| TO_DAYS() | Return the date argument converted to days |
| UNIX_TIMESTAMP() | Return a UNIX timestamp |
| UTC_DATE() | Return the current UTC date |
| UTC_TIME() | Return the current UTC time |
| UTC_TIMESTAMP() | Return the current UTC date and time |
| WEEK() | Return the week number |
| WEEKDAY() | Return the weekday index |
| WEEKOFYEAR() | Return the calendar week of the date (1-53) |
| YEAR() | Return the year |
| YEARWEEK() | Return the year and week |
ADDDATE(date,INTERVAL expr unit), ADDDATE(expr,days)
When invoked with the INTERVAL form of the second argument, ADDDATE() is a synonym for DATE_ADD(). The related function SUBDATE() is a synonym for DATE_SUB(). For information on the INTERVAL unit argument, see the discussion for DATE_ADD().
mysql> SELECT DATE_ADD('1998-01-02', INTERVAL 31 DAY); +---------------------------------------------------------+ | DATE_ADD('1998-01-02', INTERVAL 31 DAY) | +---------------------------------------------------------+ | 1998-02-02 | +---------------------------------------------------------+ 1 row in set (0.00 sec) mysql> SELECT ADDDATE('1998-01-02', INTERVAL 31 DAY); +---------------------------------------------------------+ | ADDDATE('1998-01-02', INTERVAL 31 DAY) | +---------------------------------------------------------+ | 1998-02-02 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
When invoked with the days form of the second argument, MySQL treats it as an integer number of days to be added to expr.
mysql> SELECT ADDDATE('1998-01-02', 31); +---------------------------------------------------------+ | DATE_ADD('1998-01-02', INTERVAL 31 DAY) | +---------------------------------------------------------+ | 1998-02-02 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
ADDTIME(expr1,expr2)
ADDTIME() adds expr2 to expr1 and returns the result. expr1 is a time or datetime expression, and expr2 is a time expression.
mysql> SELECT ADDTIME('1997-12-31 23:59:59.999999','1 1:1:1.000002'); +---------------------------------------------------------+ | DATE_ADD('1997-12-31 23:59:59.999999','1 1:1:1.000002') | +---------------------------------------------------------+ | 1998-01-02 01:01:01.000001 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
CONVERT_TZ(dt,from_tz,to_tz)
This converts a datetime value dt from the time zone given by from_tz to the time zone given by to_tz and returns the resulting value. This function returns NULL if the arguments are invalid.
mysql> SELECT CONVERT_TZ('2004-01-01 12:00:00','GMT','MET'); +---------------------------------------------------------+ | CONVERT_TZ('2004-01-01 12:00:00','GMT','MET') | +---------------------------------------------------------+ | 2004-01-01 13:00:00 | +---------------------------------------------------------+ 1 row in set (0.00 sec) mysql> SELECT CONVERT_TZ('2004-01-01 12:00:00','+00:00','+10:00'); +---------------------------------------------------------+ | CONVERT_TZ('2004-01-01 12:00:00','+00:00','+10:00') | +---------------------------------------------------------+ | 2004-01-01 22:00:00 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
CURDATE()
Returns the current date as a value in 'YYYY-MM-DD' or YYYYMMDD format, depending on whether the function is used in a string or numeric context.
mysql> SELECT CURDATE(); +---------------------------------------------------------+ | CURDATE() | +---------------------------------------------------------+ | 1997-12-15 | +---------------------------------------------------------+ 1 row in set (0.00 sec) mysql> SELECT CURDATE() + 0; +---------------------------------------------------------+ | CURDATE() + 0 | +---------------------------------------------------------+ | 19971215 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
CURRENT_DATE and CURRENT_DATE()
CURRENT_DATE and CURRENT_DATE() are synonyms for CURDATE()
CURTIME()
Returns the current time as a value in 'HH:MM:SS' or HHMMSS format, depending on whether the function is used in a string or numeric context. The value is expressed in the current time zone.
mysql> SELECT CURTIME(); +---------------------------------------------------------+ | CURTIME() | +---------------------------------------------------------+ | 23:50:26 | +---------------------------------------------------------+ 1 row in set (0.00 sec) mysql> SELECT CURTIME() + 0; +---------------------------------------------------------+ | CURTIME() + 0 | +---------------------------------------------------------+ | 235026 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
CURRENT_TIME and CURRENT_TIME()
CURRENT_TIME and CURRENT_TIME() are synonyms for CURTIME().
CURRENT_TIMESTAMP and CURRENT_TIMESTAMP()
CURRENT_TIMESTAMP and CURRENT_TIMESTAMP() are synonyms for NOW().
DATE(expr)
Extracts the date part of the date or datetime expression expr.
mysql> SELECT DATE('2003-12-31 01:02:03'); +---------------------------------------------------------+ | DATE('2003-12-31 01:02:03') | +---------------------------------------------------------+ | 2003-12-31 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
DATEDIFF(expr1,expr2)
DATEDIFF() returns expr1 . expr2 expressed as a value in days from one date to the other. expr1 and expr2 are date or date-and-time expressions. Only the date parts of the values are used in the calculation.
mysql> SELECT DATEDIFF('1997-12-31 23:59:59','1997-12-30'); +---------------------------------------------------------+ | DATEDIFF('1997-12-31 23:59:59','1997-12-30') | +---------------------------------------------------------+ | 1 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
DATE_ADD(date,INTERVAL expr unit), DATE_SUB(date,INTERVAL expr unit)
These functions perform date arithmetic. date is a DATETIME or DATE value specifying the starting date. expr is an expression specifying the interval value to be added or subtracted from the starting date. expr is a string; it may start with a .-. for negative intervals. unit is a keyword indicating the units in which the expression should be interpreted.
The INTERVAL keyword and the unit specifier are not case sensitive.
The following table shows the expected form of the expr argument for each unit value;
| unit Value | ExpectedexprFormat |
| MICROSECOND | MICROSECONDS |
| SECOND | SECONDS |
| MINUTE | MINUTES |
| HOUR | HOURS |
| DAY | DAYS |
| WEEK | WEEKS |
| MONTH | MONTHS |
| QUARTER | QUARTERS |
| YEAR | YEARS |
| SECOND_MICROSECOND | 'SECONDS.MICROSECONDS' |
| MINUTE_MICROSECOND | 'MINUTES.MICROSECONDS' |
| MINUTE_SECOND | 'MINUTES:SECONDS' |
| HOUR_MICROSECOND | 'HOURS.MICROSECONDS' |
| HOUR_SECOND | 'HOURS:MINUTES:SECONDS' |
| HOUR_MINUTE | 'HOURS:MINUTES' |
| DAY_MICROSECOND | 'DAYS.MICROSECONDS' |
| DAY_SECOND | 'DAYS HOURS:MINUTES:SECONDS' |
| DAY_MINUTE | 'DAYS HOURS:MINUTES' |
| DAY_HOUR | 'DAYS HOURS' |
| YEAR_MONTH | 'YEARS-MONTHS' |
The values QUARTER and WEEK are available beginning with MySQL 5.0.0.
mysql> SELECT DATE_ADD('1997-12-31 23:59:59', -> INTERVAL '1:1' MINUTE_SECOND); +---------------------------------------------------------+ | DATE_ADD('1997-12-31 23:59:59', INTERVAL... | +---------------------------------------------------------+ | 1998-01-01 00:01:00 | +---------------------------------------------------------+ 1 row in set (0.00 sec) mysql> SELECT DATE_ADD('1999-01-01', INTERVAL 1 HOUR); +---------------------------------------------------------+ | DATE_ADD('1999-01-01', INTERVAL 1 HOUR) | +---------------------------------------------------------+ | 1999-01-01 01:00:00 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
DATE_FORMAT(date,format)
Formats the date value according to the format string.
The following specifiers may be used in the format string. The .%. character is required before format specifier characters.
| Specifier | Description |
| %a | Abbreviated weekday name (Sun..Sat) |
| %b | Abbreviated month name (Jan..Dec) |
| %c | Month, numeric (0..12) |
| %D | Day of the month with English suffix (0th, 1st, 2nd, 3rd, .) |
| %d | Day of the month, numeric (00..31) |
| %e | Day of the month, numeric (0..31) |
| %f | Microseconds (000000..999999) |
| %H | Hour (00..23) |
| %h | Hour (01..12) |
| %I | Hour (01..12) |
| %i | Minutes, numeric (00..59) |
| %j | Day of year (001..366) |
| %k | Hour (0..23) |
| %l | Hour (1..12) |
| %M | Month name (January..December) |
| %m | Month, numeric (00..12) |
| %p | AM or PM |
| %r | Time, 12-hour (hh:mm:ss followed by AM or PM) |
| %S | Seconds (00..59) |
| %s | Seconds (00..59) |
| %T | Time, 24-hour (hh:mm:ss) |
| %U | Week (00..53), where Sunday is the first day of the week |
| %u | Week (00..53), where Monday is the first day of the week |
| %V | Week (01..53), where Sunday is the first day of the week; used with %X |
| %v | Week (01..53), where Monday is the first day of the week; used with %x |
| %W | Weekday name (Sunday..Saturday) |
| %w | Day of the week (0=Sunday..6=Saturday) |
| %X | Year for the week where Sunday is the first day of the week, numeric, four digits; used with %V |
| %x | Year for the week, where Monday is the first day of the week, numeric, four digits; used with %v |
| %Y | Year, numeric, four digits |
| %y | Year, numeric (two digits) |
| %% | A literal .%. character |
| %x | x, for any.x. not listed above |
mysql> SELECT DATE_FORMAT('1997-10-04 22:23:00', '%W %M %Y'); +---------------------------------------------------------+ | DATE_FORMAT('1997-10-04 22:23:00', '%W %M %Y') | +---------------------------------------------------------+ | Saturday October 1997 | +---------------------------------------------------------+ 1 row in set (0.00 sec) mysql> SELECT DATE_FORMAT('1997-10-04 22:23:00' -> '%H %k %I %r %T %S %w'); +---------------------------------------------------------+ | DATE_FORMAT('1997-10-04 22:23:00....... | +---------------------------------------------------------+ | 22 22 10 10:23:00 PM 22:23:00 00 6 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
DATE_SUB(date,INTERVAL expr unit)
This is similar to DATE_ADD() function.
DAY(date)
DAY() is a synonym for DAYOFMONTH().
DAYNAME(date)
Returns the name of the weekday for date.
mysql> SELECT DAYNAME('1998-02-05'); +---------------------------------------------------------+ | DAYNAME('1998-02-05') | +---------------------------------------------------------+ | Thursday | +---------------------------------------------------------+ 1 row in set (0.00 sec)
DAYOFMONTH(date)
Returns the day of the month for date, in the range 0 to 31.
mysql> SELECT DAYOFMONTH('1998-02-03'); +---------------------------------------------------------+ | DAYOFMONTH('1998-02-03') | +---------------------------------------------------------+ | 3 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
DAYOFWEEK(date)
Returns the weekday index for date (1 = Sunday, 2 = Monday, ., 7 = Saturday). These index values correspond to the ODBC standard.
mysql> SELECT DAYOFWEEK('1998-02-03'); +---------------------------------------------------------+ |DAYOFWEEK('1998-02-03') | +---------------------------------------------------------+ | 3 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
DAYOFYEAR(date)
Returns the day of the year for date, in the range 1 to 366.
mysql> SELECT DAYOFYEAR('1998-02-03'); +---------------------------------------------------------+ | DAYOFYEAR('1998-02-03') | +---------------------------------------------------------+ | 34 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
EXTRACT(unit FROM date)
The EXTRACT() function uses the same kinds of unit specifiers as DATE_ADD() or DATE_SUB(), but extracts parts from the date rather than performing date arithmetic.
mysql> SELECT EXTRACT(YEAR FROM '1999-07-02'); +---------------------------------------------------------+ | EXTRACT(YEAR FROM '1999-07-02') | +---------------------------------------------------------+ | 1999 | +---------------------------------------------------------+ 1 row in set (0.00 sec) mysql> SELECT EXTRACT(YEAR_MONTH FROM '1999-07-02 01:02:03'); +---------------------------------------------------------+ | EXTRACT(YEAR_MONTH FROM '1999-07-02 01:02:03') | +---------------------------------------------------------+ | 199907 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
FROM_DAYS(N)
Given a day number N, returns a DATE value.
mysql> SELECT FROM_DAYS(729669); +---------------------------------------------------------+ | FROM_DAYS(729669) | +---------------------------------------------------------+ | 1997-10-07 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
Use FROM_DAYS() with caution on old dates. It is not intended for use with values that precede the advent of the Gregorian calendar (1582).
FROM_UNIXTIME(unix_timestamp)
FROM_UNIXTIME(unix_timestamp,format)
Returns a representation of the unix_timestamp argument as a value in 'YYYY-MM-DD HH:MM:SS' or YYYYMMDDHHMMSS format, depending on whether the function is used in a string or numeric context. The value is expressed in the current time zone. unix_timestamp is an internal timestamp value such as is produced by the UNIX_TIMESTAMP() function.
If format is given, the result is formatted according to the format string, which is used the same way as listed in the entry for the DATE_FORMAT() function.
mysql> SELECT FROM_UNIXTIME(875996580); +---------------------------------------------------------+ | FROM_UNIXTIME(875996580) | +---------------------------------------------------------+ | 1997-10-04 22:23:00 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
HOUR(time)
Returns the hour for time. The range of the return value is 0 to 23 for time-of-day values. However, the range of TIME values actually is much larger, so HOUR can return values greater than 23.
mysql> SELECT HOUR('10:05:03'); +---------------------------------------------------------+ | HOUR('10:05:03') | +---------------------------------------------------------+ | 10 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
LAST_DAY(date)
Takes a date or datetime value and returns the corresponding value for the last day of the month. Returns NULL if the argument is invalid.
mysql> SELECT LAST_DAY('2003-02-05'); +---------------------------------------------------------+ | LAST_DAY('2003-02-05') | +---------------------------------------------------------+ | 2003-02-28 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
LOCALTIME and LOCALTIME()
LOCALTIME and LOCALTIME() are synonyms for NOW().
LOCALTIMESTAMP and LOCALTIMESTAMP()
LOCALTIMESTAMP and LOCALTIMESTAMP() are synonyms for NOW().
MAKEDATE(year,dayofyear)
Returns a date, given year and day-of-year values. dayofyear must be greater than 0 or the result is NULL.
mysql> SELECT MAKEDATE(2001,31), MAKEDATE(2001,32); +---------------------------------------------------------+ | MAKEDATE(2001,31), MAKEDATE(2001,32) | +---------------------------------------------------------+ | '2001-01-31', '2001-02-01' | +---------------------------------------------------------+ 1 row in set (0.00 sec)
MAKETIME(hour,minute,second)
Returns a time value calculated from the hour, minute, and second arguments.
mysql> SELECT MAKETIME(12,15,30); +---------------------------------------------------------+ | MAKETIME(12,15,30) | +---------------------------------------------------------+ | '12:15:30' | +---------------------------------------------------------+ 1 row in set (0.00 sec)
MICROSECOND(expr)
Returns the microseconds from the time or datetime expression expr as a number in the range from 0 to 999999.
mysql> SELECT MICROSECOND('12:00:00.123456'); +---------------------------------------------------------+ | MICROSECOND('12:00:00.123456') | +---------------------------------------------------------+ | 123456 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
MINUTE(time)
Returns the minute for time, in the range 0 to 59.
mysql> SELECT MINUTE('98-02-03 10:05:03'); +---------------------------------------------------------+ | MINUTE('98-02-03 10:05:03') | +---------------------------------------------------------+ | 5 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
MONTH(date)
Returns the month for date, in the range 0 to 12.
mysql> SELECT MONTH('1998-02-03') +---------------------------------------------------------+ | MONTH('1998-02-03') | +---------------------------------------------------------+ | 2 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
MONTHNAME(date)
Returns the full name of the month for date.
mysql> SELECT MONTHNAME('1998-02-05'); +---------------------------------------------------------+ | MONTHNAME('1998-02-05') | +---------------------------------------------------------+ | February | +---------------------------------------------------------+ 1 row in set (0.00 sec)
NOW()
Returns the current date and time as a value in 'YYYY-MM-DD HH:MM:SS' or YYYYMMDDHHMMSS format, depending on whether the function is used in a string or numeric context. The value is expressed in the current time zone.
mysql> SELECT NOW(); +---------------------------------------------------------+ | NOW() | +---------------------------------------------------------+ | 1997-12-15 23:50:26 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
PERIOD_ADD(P,N)
Adds N months to period P (in the format YYMM or YYYYMM). Returns a value in the format YYYYMM. Note that the period argument P is not a date value.
mysql> SELECT PERIOD_ADD(9801,2); +---------------------------------------------------------+ | PERIOD_ADD(9801,2) | +---------------------------------------------------------+ | 199803 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
PERIOD_DIFF(P1,P2)
Returns the number of months between periods P1 and P2. P1 and P2 should be in the format YYMM or YYYYMM. Note that the period arguments P1 and P2 are not date values.
mysql> SELECT PERIOD_DIFF(9802,199703); +---------------------------------------------------------+ | PERIOD_DIFF(9802,199703) | +---------------------------------------------------------+ | 11 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
QUARTER(date)
Returns the quarter of the year for date, in the range 1 to 4.
mysql> SELECT QUARTER('98-04-01'); +---------------------------------------------------------+ | QUARTER('98-04-01') | +---------------------------------------------------------+ | 2 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
SECOND(time)
Returns the second for time, in the range 0 to 59.
mysql> SELECT SECOND('10:05:03'); +---------------------------------------------------------+ | SECOND('10:05:03') | +---------------------------------------------------------+ | 3 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
SEC_TO_TIME(seconds)
Returns the seconds argument, converted to hours, minutes, and seconds, as a value in 'HH:MM:SS' or HHMMSS format, depending on whether the function is used in a string or numeric context.
mysql> SELECT SEC_TO_TIME(2378); +---------------------------------------------------------+ | SEC_TO_TIME(2378) | +---------------------------------------------------------+ | 00:39:38 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
STR_TO_DATE(str,format)
This is the inverse of the DATE_FORMAT() function. It takes a string str and a format string format. STR_TO_DATE() returns a DATETIME value if the format string contains both date and time parts, or a DATE or TIME value if the string contains only date or time parts.
mysql> SELECT STR_TO_DATE('04/31/2004', '%m/%d/%Y'); +---------------------------------------------------------+ | STR_TO_DATE('04/31/2004', '%m/%d/%Y') | +---------------------------------------------------------+ | 2004-04-31 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
SUBDATE(date,INTERVAL expr unit) and SUBDATE(expr,days)
When invoked with the INTERVAL form of the second argument, SUBDATE() is a synonym for DATE_SUB(). For information on the INTERVAL unit argument, see the discussion for DATE_ADD().
mysql> SELECT DATE_SUB('1998-01-02', INTERVAL 31 DAY); +---------------------------------------------------------+ | DATE_SUB('1998-01-02', INTERVAL 31 DAY) | +---------------------------------------------------------+ | 1997-12-02 | +---------------------------------------------------------+ 1 row in set (0.00 sec) mysql> SELECT SUBDATE('1998-01-02', INTERVAL 31 DAY); +---------------------------------------------------------+ | SUBDATE('1998-01-02', INTERVAL 31 DAY) | +---------------------------------------------------------+ | 1997-12-02 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
SUBTIME(expr1,expr2)
SUBTIME() returns expr1 . expr2 expressed as a value in the same format as expr1. expr1 is a time or datetime expression, and expr2 is a time.
mysql> SELECT SUBTIME('1997-12-31 23:59:59.999999', -> '1 1:1:1.000002'); +---------------------------------------------------------+ | SUBTIME('1997-12-31 23:59:59.999999'... | +---------------------------------------------------------+ | 1997-12-30 22:58:58.999997 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
SYSDATE()
Returns the current date and time as a value in 'YYYY-MM-DD HH:MM:SS' or YYYYMMDDHHMMSS format, depending on whether the function is used in a string or numeric context.
mysql> SELECT SYSDATE(); +---------------------------------------------------------+ | SYSDATE() | +---------------------------------------------------------+ | 2006-04-12 13:47:44 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
TIME(expr)
Extracts the time part of the time or datetime expression expr and returns it as a string.
mysql> SELECT TIME('2003-12-31 01:02:03'); +---------------------------------------------------------+ | TIME('2003-12-31 01:02:03') | +---------------------------------------------------------+ | 01:02:03 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
TIMEDIFF(expr1,expr2)
TIMEDIFF() returns expr1 . expr2 expressed as a time value. expr1 and expr2 are time or date-and-time expressions, but both must be of the same type.
mysql> SELECT TIMEDIFF('1997-12-31 23:59:59.000001', -> '1997-12-30 01:01:01.000002'); +---------------------------------------------------------+ | TIMEDIFF('1997-12-31 23:59:59.000001'..... | +---------------------------------------------------------+ | 46:58:57.999999 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
TIMESTAMP(expr), TIMESTAMP(expr1,expr2)
With a single argument, this function returns the date or datetime expression expr as a datetime value. With two arguments, it adds the time expression expr2 to the date or datetime expression expr1 and returns the result as a datetime value.
mysql> SELECT TIMESTAMP('2003-12-31'); +---------------------------------------------------------+ | TIMESTAMP('2003-12-31') | +---------------------------------------------------------+ | 2003-12-31 00:00:00 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
TIMESTAMPADD(unit,interval,datetime_expr)
Adds the integer expression interval to the date or datetime expression datetime_expr. The unit for interval is given by the unit argument, which should be one of the following values: FRAC_SECOND, SECOND, MINUTE, HOUR, DAY, WEEK, MONTH, QUARTER, or YEAR.
The unit value may be specified using one of keywords as shown, or with a prefix of SQL_TSI_. For example, DAY and SQL_TSI_DAY both are legal.
mysql> SELECT TIMESTAMPADD(MINUTE,1,'2003-01-02'); +---------------------------------------------------------+ | TIMESTAMPADD(MINUTE,1,'2003-01-02') | +---------------------------------------------------------+ | 2003-01-02 00:01:00 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
TIMESTAMPDIFF(unit,datetime_expr1,datetime_expr2)
Returns the integer difference between the date or datetime expressions datetime_expr1 and datetime_expr2. The unit for the result is given by the unit argument. The legal values for unit are the same as those listed in the description of the TIMESTAMPADD() function.
mysql> SELECT TIMESTAMPDIFF(MONTH,'2003-02-01','2003-05-01'); +---------------------------------------------------------+ | TIMESTAMPDIFF(MONTH,'2003-02-01','2003-05-01') | +---------------------------------------------------------+ | 3 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
TIME_FORMAT(time,format)
This is used like the DATE_FORMAT() function, but the format string may contain format specifiers only for hours, minutes, and seconds.
If the time value contains an hour part that is greater than 23, the %H and %k hour format specifiers produce a value larger than the usual range of 0..23. The other hour format specifiers produce the hour value modulo 12.
mysql> SELECT TIME_FORMAT('100:00:00', '%H %k %h %I %l'); +---------------------------------------------------------+ | TIME_FORMAT('100:00:00', '%H %k %h %I %l') | +---------------------------------------------------------+ | 100 100 04 04 4 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
TIME_TO_SEC(time)
Returns the time argument, converted to seconds.
mysql> SELECT TIME_TO_SEC('22:23:00'); +---------------------------------------------------------+ | TIME_TO_SEC('22:23:00') | +---------------------------------------------------------+ | 80580 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
TO_DAYS(date)
Given a date date, returns a day number (the number of days since year 0).
mysql> SELECT TO_DAYS(950501); +---------------------------------------------------------+ | TO_DAYS(950501) | +---------------------------------------------------------+ | 728779 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
UNIX_TIMESTAMP(), UNIX_TIMESTAMP(date)
If called with no argument, returns a Unix timestamp (seconds since '1970-01-01 00:00:00' UTC) as an unsigned integer. If UNIX_TIMESTAMP() is called with a date argument, it returns the value of the argument as seconds since '1970-01-01 00:00:00' UTC. date may be a DATE string, a DATETIME string, a TIMESTAMP, or a number in the format YYMMDD or YYYYMMDD.
mysql> SELECT UNIX_TIMESTAMP(); +---------------------------------------------------------+ | UNIX_TIMESTAMP() | +---------------------------------------------------------+ | 882226357 | +---------------------------------------------------------+ 1 row in set (0.00 sec) mysql> SELECT UNIX_TIMESTAMP('1997-10-04 22:23:00'); +---------------------------------------------------------+ | UNIX_TIMESTAMP('1997-10-04 22:23:00') | +---------------------------------------------------------+ | 875996580 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
UTC_DATE, UTC_DATE()
Returns the current UTC date as a value in 'YYYY-MM-DD' or YYYYMMDD format, depending on whether the function is used in a string or numeric context.
mysql> SELECT UTC_DATE(), UTC_DATE() + 0; +---------------------------------------------------------+ | UTC_DATE(), UTC_DATE() + 0 | +---------------------------------------------------------+ | 2003-08-14, 20030814 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
UTC_TIME, UTC_TIME()
Returns the current UTC time as a value in 'HH:MM:SS' or HHMMSS format, depending on whether the function is used in a string or numeric context.
mysql> SELECT UTC_TIME(), UTC_TIME() + 0; +---------------------------------------------------------+ | UTC_TIME(), UTC_TIME() + 0 | +---------------------------------------------------------+ | 18:07:53, 180753 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
UTC_TIMESTAMP, UTC_TIMESTAMP()
Returns the current UTC date and time as a value in 'YYYY-MM-DD HH:MM:SS' or YYYYMMDDHHMMSS format, depending on whether the function is used in a string or numeric context.
mysql> SELECT UTC_TIMESTAMP(), UTC_TIMESTAMP() + 0; +---------------------------------------------------------+ | UTC_TIMESTAMP(), UTC_TIMESTAMP() + 0 | +---------------------------------------------------------+ | 2003-08-14 18:08:04, 20030814180804 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
WEEK(date[,mode])
This function returns the week number for date. The two-argument form of WEEK() allows you to specify whether the week starts on Sunday or Monday and whether the return value should be in the range from 0 to 53 or from 1 to 53. If the mode argument is omitted, the value of the default_week_format system variable is used
| Mode | First Day of week | Range | Week 1 is the first week . |
| 0 | Sunday | 0-53 | with a Sunday in this year |
| 1 | Monday | 0-53 | with more than 3 days this year |
| 2 | Sunday | 1-53 | with a Sunday in this year |
| 3 | Monday | 1-53 | with more than 3 days this year |
| 4 | Sunday | 0-53 | with more than 3 days this year |
| 5 | Monday | 0-53 | with a Monday in this year |
| 6 | Sunday | 1-53 | with more than 3 days this year |
| 7 | Monday | 1-53 | with a Monday in this year |
mysql> SELECT WEEK('1998-02-20'); +---------------------------------------------------------+ | WEEK('1998-02-20') | +---------------------------------------------------------+ | 7 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
WEEKDAY(date)
Returns the weekday index for date (0 = Monday, 1 = Tuesday, . 6 = Sunday).
mysql> SELECT WEEKDAY('1998-02-03 22:23:00'); +---------------------------------------------------------+ | WEEKDAY('1998-02-03 22:23:00') | +---------------------------------------------------------+ | 1 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
WEEKOFYEAR(date)
Returns the calendar week of the date as a number in the range from 1 to 53. WEEKOFYEAR() is a compatibility function that is equivalent to WEEK(date,3).
mysql> SELECT WEEKOFYEAR('1998-02-20'); +---------------------------------------------------------+ | WEEKOFYEAR('1998-02-20') | +---------------------------------------------------------+ | 8 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
YEAR(date)
Returns the year for date, in the range 1000 to 9999, or 0 for the .zero. date.
mysql> SELECT YEAR('98-02-03'); +---------------------------------------------------------+ | YEAR('98-02-03') | +---------------------------------------------------------+ | 1998 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
YEARWEEK(date), YEARWEEK(date,mode)
Returns year and week for a date. The mode argument works exactly like the mode argument to WEEK(). The year in the result may be different from the year in the date argument for the first and the last week of the year.
mysql> SELECT YEARWEEK('1987-01-01'); +---------------------------------------------------------+ | YEAR('98-02-03')YEARWEEK('1987-01-01') | +---------------------------------------------------------+ | 198653 | +---------------------------------------------------------+ 1 row in set (0.00 sec)
Note that the week number is different from what the WEEK() function would return (0) for optional arguments 0 or 1, as WEEK() then returns the week in the context of the given year.
SQL - Temporary Tables
There are RDBMS which support temporary tables. Temporary Tables are a great feature that lets you store and process intermediate results by using the same selection, update, and join capabilities that you can use with typical SQL Server tables.
The temporary tables could be very useful in some cases to keep temporary data. The most important thing that should be knows for temporary tables is that they will be deleted when the current client session terminates.
Temporary tables are available in MySQL version 3.23 onwards. If you use an older version of MySQL than 3.23 you can't use temporary tables, but you can use heap tables.
As stated earlier temporary tables will only last as long as the session is alive. If you run the code in a PHP script, the temporary table will be destroyed automatically when the script finishes executing. If you are connected to the MySQL database server through the MySQL client program, then the temporary table will exist until you close the client or manually destroy the table.
Example
Here is an example showing you usage of temporary table:
mysql> CREATE TEMPORARY TABLE SALESSUMMARY ( -> product_name VARCHAR(50) NOT NULL -> , total_sales DECIMAL(12,2) NOT NULL DEFAULT 0.00 -> , avg_unit_price DECIMAL(7,2) NOT NULL DEFAULT 0.00 -> , total_units_sold INT UNSIGNED NOT NULL DEFAULT 0 ); Query OK, 0 rows affected (0.00 sec) mysql> INSERT INTO SALESSUMMARY -> (product_name, total_sales, avg_unit_price, total_units_sold) -> VALUES -> ('cucumber', 100.25, 90, 2); mysql> SELECT * FROM SALESSUMMARY; +--------------+-------------+----------------+------------------+ | product_name | total_sales | avg_unit_price | total_units_sold | +--------------+-------------+----------------+------------------+ | cucumber | 100.25 | 90.00 | 2 | +--------------+-------------+----------------+------------------+ 1 row in set (0.00 sec)
When you issue a SHOW TABLES command then your temporary table would not be listed out in the list. Now if you will log out of the MySQL session and then you will issue a SELECT command then you will find no data available in the database. Even your temporary table would also not exist.
Dropping Temporary Tables:
By default all the temporary tables are deleted by MySQL when your database connection gets terminated. Still you want to delete them in between then you do so by issuing DROP TABLE command.
Following is the example on dropping a temproary table.
mysql> CREATE TEMPORARY TABLE SALESSUMMARY ( -> product_name VARCHAR(50) NOT NULL -> , total_sales DECIMAL(12,2) NOT NULL DEFAULT 0.00 -> , avg_unit_price DECIMAL(7,2) NOT NULL DEFAULT 0.00 -> , total_units_sold INT UNSIGNED NOT NULL DEFAULT 0 ); Query OK, 0 rows affected (0.00 sec) mysql> INSERT INTO SALESSUMMARY -> (product_name, total_sales, avg_unit_price, total_units_sold) -> VALUES -> ('cucumber', 100.25, 90, 2); mysql> SELECT * FROM SALESSUMMARY; +--------------+-------------+----------------+------------------+ | product_name | total_sales | avg_unit_price | total_units_sold | +--------------+-------------+----------------+------------------+ | cucumber | 100.25 | 90.00 | 2 | +--------------+-------------+----------------+------------------+ 1 row in set (0.00 sec) mysql> DROP TABLE SALESSUMMARY; mysql> SELECT * FROM SALESSUMMARY; ERROR 1146: Table 'TUTORIALS.SALESSUMMARY' doesn't exist
SQL - Clone Tables
There may be a situation when you need an exact copy of a table, and CREATE TABLE ... SELECT... doesn't suit your purposes because the copy must include the same indexes, default values, and so forth.
If you are using MySQL RDBMS, you can handle this situation by following steps.
- Use SHOW CREATE TABLE command to get a CREATE TABLE statement that specifies the source table's structure, indexes and all.
- Modify the statement to change the table name to that of the clone table and execute the statement. This way you will have exact clone table.
- Optionally, If you need the table contents copied as well, issue an INSERT INTO ... SELECT statement, too.
Example:
Try out following example to create a clone table for TUTORIALS_TBL whose structure is as follows:
Step 1:
Get complete structure about table
SQL> SHOW CREATE TABLE TUTORIALS_TBL \G; *************************** 1. row *************************** Table: TUTORIALS_TBL Create Table: CREATE TABLE `TUTORIALS_TBL` ( `tutorial_id` int(11) NOT NULL auto_increment, `tutorial_title` varchar(100) NOT NULL default '', `tutorial_author` varchar(40) NOT NULL default '', `submission_date` date default NULL, PRIMARY KEY (`tutorial_id`), UNIQUE KEY `AUTHOR_INDEX` (`tutorial_author`) ) TYPE=MyISAM 1 row in set (0.00 sec)
Step 2:
Rename this table and create another table
SQL> CREATE TABLE `CLONE_TBL` ( -> `tutorial_id` int(11) NOT NULL auto_increment, -> `tutorial_title` varchar(100) NOT NULL default '', -> `tutorial_author` varchar(40) NOT NULL default '', -> `submission_date` date default NULL, -> PRIMARY KEY (`tutorial_id`), -> UNIQUE KEY `AUTHOR_INDEX` (`tutorial_author`) -> ) TYPE=MyISAM; Query OK, 0 rows affected (1.80 sec)
Step 3:
After executing step 2 you will a clone table in your database. If you want to copy data from old table then you can do it by using INSERT INTO... SELECT statement.
SQL> INSERT INTO CLONE_TBL (tutorial_id, -> tutorial_title, -> tutorial_author, -> submission_date) -> SELECT tutorial_id,tutorial_title, -> tutorial_author,submission_date, -> FROM TUTORIALS_TBL; Query OK, 3 rows affected (0.07 sec) Records: 3 Duplicates: 0 Warnings: 0
Finally you will have exact clone table as you wanted to have.
SQL - Sub Queries
A Subquery or Inner query or Nested query is a query within another SQL query, and embedded within the WHERE clause.
A subquery is used to return data that will be used in the main query as a condition to further restrict the data to be retrieved.
Subqueries can be used with the SELECT, INSERT, UPDATE, and DELETE statements along with the operators like =, <, >, >=, <=, IN, BETWEEN etc.
There are a few rules that subqueries must follow:
- Subqueries must be enclosed within parentheses.
- A subquery can have only one column in the SELECT clause, unless multiple columns are in the main query for the subquery to compare its selected columns.
- An ORDER BY cannot be used in a subquery, although the main query can use an ORDER BY. The GROUP BY can be used to perform the same function as the ORDER BY in a subquery.
- Subqueries that return more than one row can only be used with multiple value operators, such as the IN operator.
- The SELECT list cannot include any references to values that evaluate to a BLOB, ARRAY, CLOB, or NCLOB.
- A subquery cannot be immediately enclosed in a set function.
- The BETWEEN operator cannot be used with a subquery; however, the BETWEEN can be used within the subquery.
Subqueries with the SELECT Statement:
Subqueries are most frequently used with the SELECT statement. The basic syntax is as follows:
SELECT column_name [, column_name ] FROM table1 [, table2 ] WHERE column_name OPERATOR (SELECT column_name [, column_name ] FROM table1 [, table2 ] [WHERE])
Example:
Consider CUSTOMERS table is having following records:
+----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 1 | Ramesh | 35 | Ahmedabad | 2000.00 | | 2 | Khilan | 25 | Delhi | 1500.00 | | 3 | kaushik | 23 | Kota | 2000.00 | | 4 | Chaitali | 25 | Mumbai | 6500.00 | | 5 | Hardik | 27 | Bhopal | 8500.00 | | 6 | Komal | 22 | MP | 4500.00 | | 7 | Muffy | 24 | Indore | 10000.00 | +----+----------+-----+-----------+----------+
Now let us check following sub-query with SELECT statement:
SQL> SELECT * FROM CUSTOMERS WHERE ID IN (SELECT ID FROM CUSTOMERS WHERE SALARY > 4500) ;
This would produce following result:
+----+----------+-----+---------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+---------+----------+ | 4 | Chaitali | 25 | Mumbai | 6500.00 | | 5 | Hardik | 27 | Bhopal | 8500.00 | | 7 | Muffy | 24 | Indore | 10000.00 | +----+----------+-----+---------+----------+
Subqueries with the INSERT Statement:
Subqueries also can be used with INSERT statements. The INSERT statement uses the data returned from the subquery to insert into another table. The selected data in the subquery can be modified with any of the character, date, or number functions.
The basic syntax is as follows:
INSERT INTO table_name [ (column1 [, column2 ]) ] SELECT [ *|column1 [, column2 ] FROM table1 [, table2 ] [ WHERE VALUE OPERATOR ]
Example:
Consider a table CUSTOMERS_BKP with similar structure as CUSTOMERS table. Now to copy complete CUSTOMERS table into CUSTOMERS_BKP, following is the syntax:
SQL> INSERT INTO CUSTOMERS_BKP SELECT * FROM CUSTOMERS WHERE ID IN (SELECT ID FROM CUSTOMERS) ;
Subqueries with the UPDATE Statement:
The subquery can be used in conjunction with the UPDATE statement. Either single or multiple columns in a table can be updated when using a subquery with the UPDATE statement.
The basic syntax is as follows:
UPDATE table SET column_name = new_value [ WHERE OPERATOR [ VALUE ] (SELECT COLUMN_NAME FROM TABLE_NAME) [ WHERE) ]
Example:
Assuming, we have CUSTOMERS_BKP table available which is backup of CUSTOMERS table.
Following example updates SALARY by 0.25 times in CUSTOMERS table for all the customers whose AGE is greater than or equal to 27:
SQL> UPDATE CUSTOMERS SET SALARY = SALARY * 0.25 WHERE AGE IN (SELECT AGE FROM CUSTOMERS_BKP WHERE AGE >= 27 );
This would impact two rows and finally CUSTOMERS table would have following records:
+----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 1 | Ramesh | 35 | Ahmedabad | 125.00 | | 2 | Khilan | 25 | Delhi | 1500.00 | | 3 | kaushik | 23 | Kota | 2000.00 | | 4 | Chaitali | 25 | Mumbai | 6500.00 | | 5 | Hardik | 27 | Bhopal | 2125.00 | | 6 | Komal | 22 | MP | 4500.00 | | 7 | Muffy | 24 | Indore | 10000.00 | +----+----------+-----+-----------+----------+
Subqueries with the DELETE Statement:
The subquery can be used in conjunction with the DELETE statement like with any other statements mentioned above.
The basic syntax is as follows:
DELETE FROM TABLE_NAME [ WHERE OPERATOR [ VALUE ] (SELECT COLUMN_NAME FROM TABLE_NAME) [ WHERE) ]
Example:
Assuming, we have CUSTOMERS_BKP table available which is backup of CUSTOMERS table.
Following example deletes records from CUSTOMERS table for all the customers whose AGE is greater than or equal to 27:
SQL> DELETE FROM CUSTOMERS WHERE AGE IN (SELECT AGE FROM CUSTOMERS_BKP WHERE AGE > 27 );
This would impact two rows and finally CUSTOMERS table would have following records:
+----+----------+-----+---------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+---------+----------+ | 2 | Khilan | 25 | Delhi | 1500.00 | | 3 | kaushik | 23 | Kota | 2000.00 | | 4 | Chaitali | 25 | Mumbai | 6500.00 | | 6 | Komal | 22 | MP | 4500.00 | | 7 | Muffy | 24 | Indore | 10000.00 | +----+----------+-----+---------+----------+
SQL - Using Sequences
A sequence is a set of integers 1, 2, 3, ... that are generated in order on demand. Sequences are frequently used in databases because many applications require each row in a table to contain a unique value, and sequences provide an easy way to generate them.
This chapter describes how to use sequences in MySQL.
Using AUTO_INCREMENT column:
The simplest way in MySQL to use Sequences is to define a column as AUTO_INCREMENT and leave rest of the things to MySQL to take care.
Example:
Try out following example. This will create table and after that it will insert few rows in this table where it is not required to give record ID because its auto incremented by MySQL.
mysql> CREATE TABLE INSECT -> ( -> id INT UNSIGNED NOT NULL AUTO_INCREMENT, -> PRIMARY KEY (id), -> name VARCHAR(30) NOT NULL, # type of insect -> date DATE NOT NULL, # date collected -> origin VARCHAR(30) NOT NULL # where collected ); Query OK, 0 rows affected (0.02 sec) mysql> INSERT INTO INSECT (id,name,date,origin) VALUES -> (NULL,'housefly','2001-09-10','kitchen'), -> (NULL,'millipede','2001-09-10','driveway'), -> (NULL,'grasshopper','2001-09-10','front yard'); Query OK, 3 rows affected (0.02 sec) Records: 3 Duplicates: 0 Warnings: 0 mysql> SELECT * FROM INSECT ORDER BY id; +----+-------------+------------+------------+ | id | name | date | origin | +----+-------------+------------+------------+ | 1 | housefly | 2001-09-10 | kitchen | | 2 | millipede | 2001-09-10 | driveway | | 3 | grasshopper | 2001-09-10 | front yard | +----+-------------+------------+------------+ 3 rows in set (0.00 sec)
Obtain AUTO_INCREMENT Values:
LAST_INSERT_ID( ) is a SQL function, so you can use it from within any client that understands how to issue SQL statements. otherwise PERL and PHH scripts provide exclusive functions to retrieve auto incremented value of last record.
PERL Example:
Use the mysql_insertid attribute to obtain the AUTO_INCREMENT value generated by a query. This attribute is accessed through either a database handle or a statement handle, depending on how you issue the query. The following example references it through the database handle:
$dbh->do ("INSERT INTO INSECT (name,date,origin) VALUES('moth','2001-09-14','windowsill')"); my $seq = $dbh->{mysql_insertid};
PHP Example:
After issuing a query that generates an AUTO_INCREMENT value, retrieve the value by calling mysql_insert_id( ):
mysql_query ("INSERT INTO INSECT (name,date,origin) VALUES('moth','2001-09-14','windowsill')", $conn_id); $seq = mysql_insert_id ($conn_id);
Renumbering an Existing Sequence:
There may be a case when you have deleted many records from a table and you want to resequence all the records. This can be done by using a simple trick but you should be very careful to do so if your table is having join with other table.
If you determine that resequencing an AUTO_INCREMENT column is unavoidable, the way to do it is to drop the column from the table, then add it again. The following example shows how to renumber the id values in the insect table using this technique:
mysql> ALTER TABLE INSECT DROP id; mysql> ALTER TABLE insect -> ADD id INT UNSIGNED NOT NULL AUTO_INCREMENT FIRST, -> ADD PRIMARY KEY (id);
Starting a Sequence at a Particular Value:
By default MySQL will start sequence from 1 but you can specify any other number as well at the time of table creation. Following is the example where MySQL will start sequence from 100.
mysql> CREATE TABLE INSECT -> ( -> id INT UNSIGNED NOT NULL AUTO_INCREMENT = 100, -> PRIMARY KEY (id), -> name VARCHAR(30) NOT NULL, # type of insect -> date DATE NOT NULL, # date collected -> origin VARCHAR(30) NOT NULL # where collected );
Alternatively, you can create the table and then set the initial sequence value with ALTER TABLE.
mysql> ALTER TABLE t AUTO_INCREMENT = 100;
SQL - Handling Duplicates
There may be a situation when you have multiple duplicate records in a table. While fetching such records, it makes more sense to fetch only unique records instead of fetching duplicate records.
The SQL DISTINCT keyword, which we already have discussed, is used in conjunction with SELECT statement to eliminate all the duplicate records and fetching only unique records.
Syntax:
The basic syntax of DISTINCT keyword to eliminate duplicate records is as follows:
SELECT DISTINCT column1, column2,.....columnN FROM table_name WHERE [condition]
Example:
Consider CUSTOMERS table is having following records:
+----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 1 | Ramesh | 32 | Ahmedabad | 2000.00 | | 2 | Khilan | 25 | Delhi | 1500.00 | | 3 | kaushik | 23 | Kota | 2000.00 | | 4 | Chaitali | 25 | Mumbai | 6500.00 | | 5 | Hardik | 27 | Bhopal | 8500.00 | | 6 | Komal | 22 | MP | 4500.00 | | 7 | Muffy | 24 | Indore | 10000.00 | +----+----------+-----+-----------+----------+
First let us see how the following SELECT query returns duplicate salary records:
SQL> SELECT SALARY FROM CUSTOMERS ORDER BY SALARY;
This would produce following result where salary 2000 is coming twice which is a duplicate record from the original table.
+----------+ | SALARY | +----------+ | 1500.00 | | 2000.00 | | 2000.00 | | 4500.00 | | 6500.00 | | 8500.00 | | 10000.00 | +----------+
Now let us use DISTINCT keywork with the above SELECT query and see the result:
SQL> SELECT DISTINCT SALARY FROM CUSTOMERS ORDER BY SALARY;
This would produce following result where we do not have any duplicate entry:
+----------+ | SALARY | +----------+ | 1500.00 | | 2000.00 | | 4500.00 | | 6500.00 | | 8500.00 | | 10000.00 | +----------+
SQL - Injection
If you take user input through a webpage and insert it into a SQL database there's a chance that you have left yourself wide open for a security issue known as SQL Injection.
This lesson will teach you how to help prevent this from happening and help you secure your scripts and SQL statements in your server side scripts such as PERL Script.
Injection usually occurs when you ask a user for input, like their name, and instead of a name they give you a SQL statement that you will unknowingly run on your database.
Never trust user provided data, process this data only after validation; as a rule, this is done by pattern matching.
In the example below, the name is restricted to alphanumerical chars plus underscore and to a length between 8 and 20 chars (Modify these rules as needed).
if (preg_match("/^\w{8,20}$/", $_GET['username'], $matches))
{
$result = mysql_query("SELECT * FROM CUSTOMERS
WHERE name=$matches[0]");
}
else
{
echo "user name not accepted";
}
To demonstrate the problem, consider this excerpt:
// supposed input
$name = "Qadir'; DELETE FROM CUSTOMERS;";
mysql_query("SELECT * FROM CUSTOMSRS WHERE name='{$name}'");
The function call is supposed to retrieve a record from the CUSTOMERS table where the name column matches the name specified by the user. Under normal circumstances, $name would only contain alphanumeric characters and perhaps spaces, such as the string ilia. But here, by appending an entirely new query to $name, the call to the database turns into disaster: the injected DELETE query removes all records from CUSTOMERS.
Fortunately, if you use MySQL, the mysql_query() function does not permit query stacking, or executing multiple SQL queries in a single function call. If you try to stack queries, the call fails.
However, other PHP database extensions, such as SQLite and PostgreSQL, happily perform stacked queries, executing all of the queries provided in one string and creating a serious security problem.
Preventing SQL Injection:
You can handle all escape characters smartly in scripting languages like PERL and PHP. The MySQL extension for PHP provides the function mysql_real_escape_string() to escape input characters that are special to MySQL.
if (get_magic_quotes_gpc())
{
$name = stripslashes($name);
}
$name = mysql_real_escape_string($name);
mysql_query("SELECT * FROM CUSTOMERS WHERE name='{$name}'");
The LIKE Quandary:
To address the LIKE quandary, a custom escaping mechanism must convert user-supplied % and _ characters to literals. Use addcslashes(), a function that let's you specify a character range to escape.
$sub = addcslashes(mysql_real_escape_string("%str"), "%_");
// $sub == \%str\_
mysql_query("SELECT * FROM messages
WHERE subject LIKE '{$sub}%'");
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