MySQL Database Triggers: An In-Depth Guide

What Are Database Triggers?

Triggers are special stored programs that are automatically executed when specific events occur in a database table. They are primarily used to maintain data integrity, enforce business rules, audit changes, and automate processes.

Trigger Fundamentals

Trigger Syntax

DELIMITER //
CREATE TRIGGER trigger_name
{BEFORE | AFTER} {INSERT | UPDATE | DELETE} ON table_name
FOR EACH ROW
BEGIN
    -- trigger body statements
END //
DELIMITER ;

Trigger Components

1. Timing: BEFORE or AFTER the event
2. Event: INSERT, UPDATE, or DELETE operation
3. Table: The table the trigger is associated with
4. Scope: Always FOR EACH ROW in MySQL (row-level triggers)
5. Body: SQL statements that execute when the trigger fires

Accessing Row Data

MySQL provides special references for accessing row data:

• NEW: Refers to the new row being inserted or the updated values in an UPDATE
• OLD: Refers to the existing row being updated or deleted

Types of Triggers

BEFORE Triggers

BEFORE triggers execute before the actual DML operation occurs. They're useful for:

• Validating or modifying input data