totn MySQL

MySQL: BEFORE DELETE Trigger

This MySQL tutorial explains how to create a BEFORE DELETE Trigger in MySQL with syntax and examples.

Description

A BEFORE DELETE Trigger means that MySQL will fire this trigger before the DELETE operation is executed.

Syntax

The syntax to create a BEFORE DELETE Trigger in MySQL is:

CREATE TRIGGER trigger_name
BEFORE DELETE
   ON table_name FOR EACH ROW

BEGIN

   -- variable declarations

   -- trigger code

END;

Parameters or Arguments

trigger_name
The name of the trigger to create.
BEFORE DELETE
It ndicates that the trigger will fire before the DELETE operation is executed.
table_name
The name of the table that the trigger is created on.

Restrictions

  • You can not create a BEFORE trigger on a view.
  • You can update the NEW values.
  • You can not update the OLD values.

Note

Example

Let's look at an example of how to create an BEFORE DELETE trigger using the CREATE TRIGGER statement in MySQL.

If you had a table created as follows:

CREATE TABLE contacts
( contact_id INT(11) NOT NULL AUTO_INCREMENT,
  last_name VARCHAR(30) NOT NULL,
  first_name VARCHAR(25),
  birthday DATE,
  created_date DATE,
  created_by VARCHAR(30),
  CONSTRAINT contacts_pk PRIMARY KEY (contact_id)
);

We could then use the CREATE TRIGGER statement to create an BEFORE DELETE trigger as follows:

DELIMITER //

CREATE TRIGGER contacts_before_delete
BEFORE DELETE
   ON contacts FOR EACH ROW

BEGIN

   DECLARE vUser varchar(50);

   -- Find username of person performing the DELETE into table
   SELECT USER() INTO vUser;

   -- Insert record into audit table
   INSERT INTO contacts_audit
   ( contact_id,
     deleted_date,
     deleted_by)
   VALUES
   ( OLD.contact_id,
     SYSDATE(),
     vUser );

END; //

DELIMITER ;