If you use the InnoDB storage engine, there is no need for a trigger. You can achieve the same effect using a foreign key with an appropriate cascading action:
ALTER TABLE users_dir ADD FOREIGN KEY (userId) REFERENCES users (userId) ON DELETE CASCADE;
If you use MyISAM or one of the other engines that does not yet enforce foreign key constraints, you can use a trigger:
CREATE TRIGGER users_AD AFTER DELETE ON users FOR EACH ROW DELETE FROM users_dir WHERE userId = OLD.userId;
Note that the foreign key is defined on the users_dir table, but the trigger is defined on the users table.
The examples shown here for both foreign keys and triggers only address behavior if the user is deleted. You will need to decide what action to take if the user's id is updated to a different value instead.
This was first published in June 2007