SQL · Chapter 39 of 42

SQL Triggers

A TRIGGER is code that runs AUTOMATICALLY in response to an event (INSERT/UPDATE/DELETE) on a table.

Use cases: auditing, keeping derived columns in sync, enforcing complex rules.

Example 1 (sql)

-- Log every user delete
CREATE TRIGGER log_deletes
AFTER DELETE ON users
FOR EACH ROW
INSERT INTO audit_log(user_id, action) VALUES (OLD.id, 'delete');

Output

Trigger created

Runs after every DELETE on users.

Example 2 (sql)

-- Auto-update updated_at (Postgres)
CREATE TRIGGER set_updated_at
BEFORE UPDATE ON users
FOR EACH ROW EXECUTE FUNCTION update_timestamp();

Output

Trigger created

Common pattern to keep an updated_at column current.

Key points

Runs automatically on events.
BEFORE or AFTER the event.
FOR EACH ROW or FOR EACH STATEMENT.
Powerful but easy to over-use — makes debugging harder.

💡 Note: Triggers happen behind the scenes. A junior developer may not know why 'INSERT' is also touching 3 other tables. Document them well.

SQL Triggers

Key points

📝 Quick Quiz