What is a trigger?

Learn routines that fire on an event.

What is a trigger?

A trigger is code that runs automatically on an event. Fires on INSERT, UPDATE or DELETE.

CREATE TRIGGER log_insert
AFTER INSERT ON orders
FOR EACH ROW
BEGIN
  INSERT INTO log VALUES (NEW.id);
END;

This query logs on every insert.

The trigger log_insert is created.

What is a trigger? — details

A trigger runs automatically on table events. It is not called manually.

When an INSERT, UPDATE or DELETE happens, the database runs the trigger itself; application code can't forget it, which makes triggers reliable for enforcing rules.

Exercise What is a trigger?

Create a log_insert trigger: log on AFTER INSERT.

Show example solution

CREATE TRIGGER log_insert
AFTER INSERT ON orders
FOR EACH ROW
BEGIN
  INSERT INTO log VALUES (NEW.id);
END

Test yourself

When does a trigger run?
- Only on CREATE
- When called manually
- Never
- Automatically, on an event
Show answer

Automatically, on an event — A trigger runs automatically on an event.

Add TRIGGER

CREATE ____ log_insert
AFTER INSERT ON orders
FOR EACH ROW
BEGIN
  INSERT INTO log VALUES (NEW.id);
END;

Show answer

CREATE TRIGGER log_insert
AFTER INSERT ON orders
FOR EACH ROW
BEGIN
  INSERT INTO log VALUES (NEW.id);
END;

What is a trigger: match the terms

trigger

trigger
AFTER

after
BEFORE

before
INSERT

insert

Next lesson: BEFORE and AFTER

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