DevTut

# TRIGGERS

# Basic Trigger

Create Table

mysql> CREATE TABLE account (acct_num INT, amount DECIMAL(10,2));
Query OK, 0 rows affected (0.03 sec)

Create Trigger

mysql> CREATE TRIGGER ins_sum BEFORE INSERT ON account
    -> FOR EACH ROW SET @sum = @sum + NEW.amount;
Query OK, 0 rows affected (0.06 sec)

The CREATE TRIGGER statement creates a trigger named ins_sum that is associated with the account table. It also includes clauses that specify the trigger action time, the triggering event, and what to do when the trigger activates

Insert Value

To use the trigger, set the accumulator variable (@sum) to zero, execute an INSERT statement, and then see what value the variable has afterward:

mysql> SET @sum = 0;
mysql> INSERT INTO account VALUES(137,14.98),(141,1937.50),(97,-100.00);
mysql> SELECT @sum AS 'Total amount inserted';
+-----------------------+
| Total amount inserted |
+-----------------------+
| 1852.48               |
+-----------------------+

In this case, the value of @sum after the INSERT statement has executed is 14.98 + 1937.50 - 100, or 1852.48.

Drop Trigger

mysql> DROP TRIGGER test.ins_sum;

If you drop a table, any triggers for the table are also dropped.

# Types of triggers

# Timing

There are two trigger action time modifiers :

  • BEFORE trigger activates before executing the request,
  • AFTER trigger fire after change.

# Triggering event

There are three events that triggers can be attached to:

  • INSERT
  • UPDATE
  • DELETE

# Before Insert trigger example

DELIMITER $$

CREATE TRIGGER insert_date 
    BEFORE INSERT ON stack
    FOR EACH ROW
BEGIN
        -- set the insert_date field in the request before the insert
    SET NEW.insert_date = NOW();
END;

$$
DELIMITER ;

# Before Update trigger example

DELIMITER $$

CREATE TRIGGER update_date 
    BEFORE UPDATE ON stack
    FOR EACH ROW
BEGIN
        -- set the update_date field in the request before the update
    SET NEW.update_date = NOW();
END;

$$
DELIMITER ;

# After Delete trigger example

DELIMITER $$

CREATE TRIGGER deletion_date 
    AFTER DELETE ON stack
    FOR EACH ROW
BEGIN
        -- add a log entry after a successful delete
    INSERT INTO log_action(stack_id, deleted_date) VALUES(OLD.id, NOW());
END;

$$
DELIMITER ;

# Syntax

  • CREATE [DEFINER = { user | CURRENT_USER }] TRIGGER trigger_name trigger_time trigger_event ON tbl_name FOR EACH ROW [trigger_order] trigger_body
    • - trigger_time: { BEFORE | AFTER } - trigger_event: { INSERT | UPDATE | DELETE } - trigger_order: { FOLLOWS | PRECEDES } other_trigger_name

    # Remarks

    Two points need to draw your attention if you already use triggers on others DB :

    # FOR EACH ROW

    FOR EACH ROW is a mandatory part of the syntax

    You can't make a statement trigger (once by query) like Oracle do. It's more a performance related issue than a real missing feature

    # CREATE OR REPLACE TRIGGER

    The CREATE OR REPLACE is not supported by MySQL

    MySQL don't allow this syntax, you have instead to use the following :

    DELIMITER $$

DROP TRIGGER IF EXISTS myTrigger;
$$
CREATE TRIGGER myTrigger
-- ...

$$
DELIMITER ;

    Be careful, this is not an atomic transaction :

    • you'll loose the old trigger if the CREATE fail
    • on a heavy load, others operations can occurs between the DROP and the CREATE, use a LOCK TABLES myTable WRITE; first to avoid data inconsistency and UNLOCK TABLES; after the CREATE to release the table

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