How to Create Event Triggers in MySQL

How do I create BEFORE and AFTER event triggers in MySQL?

Event triggers automatically run SQL before or after INSERT, UPDATE, or DELETE statements on a table.

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Description

What are event triggers in MySQL?

MySQL triggers are stored programs that fire automatically before or after INSERT, UPDATE, or DELETE events on a table. They help you enforce business rules, maintain audit logs, and keep related data in sync without changing application code.

Why use a BEFORE trigger?

Use BEFORE triggers when you must validate or transform incoming data before it reaches the table.Examples include setting default dates, normalizing text, or rejecting invalid totals.

When is an AFTER trigger better?

AFTER triggers run once the row is safely stored, making them ideal for writing to audit tables, updating aggregates, or sending notifications that depend on committed data.

How do I create a trigger that updates product stock?

Create an AFTER INSERT trigger on OrderItems.Each time a new order item is saved, the trigger subtracts the sold quantity from Products.stock, keeping inventory accurate.

CREATE TRIGGER oi_ai_update_stock AFTER INSERT ON OrderItems FOR EACH ROW UPDATE Products SET stock = stock - NEW.quantity WHERE id = NEW.product_id;

What is the exact syntax?

The complete CREATE TRIGGER syntax lets you choose timing, event, order, and body. See the full syntax section below.

Best practices for event triggers?

Keep trigger logic short, use explicit column lists, handle errors gracefully, and document side effects.Test thoroughly because triggers run invisibly to calling code.

How do I remove or alter a trigger?

Use DROP TRIGGER [schema.]trigger_name; to delete. To change logic, drop and recreate; MySQL does not support ALTER TRIGGER.

What common mistakes should I avoid?

Do not perform long-running queries inside a trigger—this slows every DML statement.Also avoid referencing tables that may cause recursive trigger calls unless you disable the second trigger.

Quick reference table

Timing: BEFORE | AFTER
Event: INSERT | UPDATE | DELETE
Scope: FOR EACH ROW
Order: FOLLOWS | PRECEDES other_trigger

Step-by-step trigger creation

1. Decide timing and event.
2. Write the SQL body.
3. Grant TRIGGER privilege.
4. Execute CREATE TRIGGER statement.
5. Test with sample data.

Further resources?

Consult the MySQL Reference Manual, section "Triggers", for additional restrictions, such as disallowed statements and concurrency considerations.

Why How to Create Event Triggers in MySQL is important

How to Create Event Triggers in MySQL Example Usage


-- Automatically decrease product stock when an order item is added
CREATE TRIGGER oi_ai_update_stock
AFTER INSERT ON OrderItems
FOR EACH ROW
  UPDATE Products
  SET stock = stock - NEW.quantity
  WHERE id = NEW.product_id;

How to Create Event Triggers in MySQL Syntax


CREATE TRIGGER trigger_name
    BEFORE | AFTER INSERT | UPDATE | DELETE
    ON table_name
    FOR EACH ROW
    [FOLLOWS | PRECEDES other_trigger]
    trigger_body;

Example (e-commerce):
CREATE TRIGGER orders_bi_set_date
BEFORE INSERT ON Orders
FOR EACH ROW
  SET NEW.order_date = NOW();

Common Mistakes

Using long, multi-statement procedures inside a trigger slows every DML operation. Fix by moving heavy logic to stored procedures or background jobs.
Forgetting to grant TRIGGER privileges leads to permission errors. Grant with: GRANT TRIGGER ON db.* TO 'user'@'host';