Triggers in MySQL

CREATE

    [DEFINER = { user | CURRENT_USER }]

    TRIGGER trigger_name trigger_time trigger_event

    ON tbl_name FOR EACH ROW trigger_body

This statement creates a new trigger. A trigger is a named database object that is associated with a table, and that activates when a particular event occurs for the table. The trigger becomes associated with the table namedtbl_name, which must refer to a permanent table. You cannot associate a trigger with a TEMPORARY table or a view.

The DEFINER clause determines the security context to be used when checking access privileges at trigger activation time. It was added in MySQL 5.0.17.

trigger_time is the trigger action time. It can be BEFORE or AFTER to indicate that the trigger activates before or after each row to be modified.

trigger_event indicates the kind of statement that activates the trigger. The trigger_event can be one of the following:

• INSERT: The trigger is activated whenever a new row is inserted into the table; for example, through INSERT, LOAD DATA, and REPLACE statements.
• UPDATE: The trigger is activated whenever a row is modified; for example, through UPDATE statements.
• DELETE: The trigger is activated whenever a row is deleted from the table; for example, through DELETE and REPLACE statements. However, DROP TABLE and TRUNCATE TABLE statements on the table do not activate this trigger, because they do not use DELETE.
  
  

It is important to understand that the trigger_event does not represent a literal type of SQL statement that activates the trigger so much as it represents a type of table operation. 

For example, an INSERT trigger is activated by not only INSERT statements but also LOAD DATA statements because both statements insert rows into a table.

A potentially confusing example of this is the INSERT INTO ... ON DUPLICATE KEY UPDATE ... syntax: a BEFORE INSERT trigger will activate for every row, followed by either an AFTER INSERT trigger or both the BEFORE UPDATE and AFTER UPDATE triggers, depending on whether there was a duplicate key for the row.

There cannot be two triggers for a given table that have the same trigger action time and event. For example, you cannot have two BEFORE UPDATE triggers for a table. But you can have a BEFORE UPDATE and a BEFORE INSERTtrigger, or a BEFORE UPDATE and an AFTER UPDATE trigger.

trigger_body is the statement to execute when the trigger activates. If you want to execute multiple statements, use the BEGIN ... END compound statement construct. This also enables you to use the same statements that are permissible within stored routines.

You can refer to columns in the subject table (the table associated with the trigger) by using the aliases OLD and NEW. OLD.col_name refers to a column of an existing row before it is updated or deleted. NEW.col_name refers to the column of a new row to be inserted or an existing row after it is updated.

MySQL stores the sql_mode system variable setting that is in effect at the time a trigger is created, and always executes the trigger with this setting in force, regardless of the current server SQL mode.

Example:

CREATE TABLE test1(a1 INT);

CREATE TABLE test2(a2 INT);

CREATE TABLE test3(a3 INT NOT NULL AUTO_INCREMENT PRIMARY KEY);

CREATE TABLE test4(

  a4 INT NOT NULL AUTO_INCREMENT PRIMARY KEY,

  b4 INT DEFAULT 0

);

delimiter $$

CREATE TRIGGER testref BEFORE INSERT ON test1

  FOR EACH ROW BEGIN

    INSERT INTO test2 SET a2 = NEW.a1;

    DELETE FROM test3 WHERE a3 = NEW.a1;

    UPDATE test4 SET b4 = b4 + 1 WHERE a4 = NEW.a1;

  END $$

delimiter ;

INSERT INTO test3 (a3) VALUES

  (NULL), (NULL), (NULL), (NULL), (NULL),

  (NULL), (NULL), (NULL), (NULL), (NULL);

INSERT INTO test4 (a4) VALUES

  (0), (0), (0), (0), (0), (0), (0), (0), (0), (0);

Suppose that you insert the following values into table test1 as shown here:

mysql> INSERT INTO test1 VALUES (1), (3), (1), (7), (1), (8), (4), (4);

Query OK, 8 rows affected (0.01 sec)

Records: 8  Duplicates: 0  Warnings: 0

As a result, the data in the four tables will be as follows:

mysql> SELECT * FROM test1;

+------+

| a1   |

+------+

|    1 |

|    3 |

|    1 |

|    7 |

|    1 |

|    8 |

|    4 |

|    4 |

+------+

8 rows in set (0.00 sec)

mysql> SELECT * FROM test2;

+------+

| a2   |

+------+

|    1 |

|    3 |

|    1 |

|    7 |

|    1 |

|    8 |

|    4 |

|    4 |

+------+

8 rows in set (0.00 sec)

mysql> SELECT * FROM test3;

+----+

| a3 |

+----+

|  2 |

|  5 |

|  6 |

|  9 |

| 10 |

+----+

5 rows in set (0.00 sec)

mysql> SELECT * FROM test4;

+----+------+

| a4 | b4   |

+----+------+

|  1 |    3 |

|  2 |    0 |

|  3 |    1 |

|  4 |    2 |

|  5 |    0 |

|  6 |    0 |

|  7 |    1 |

|  8 |    1 |

|  9 |    0 |

| 10 |    0 |

+----+------+

10 rows in set (0.00 sec)