DELETE triggers in MySQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a DELETE trigger in MySQL?

A DELETE trigger is a special procedure that automatically runs before or after a row is deleted from a table. It helps to perform actions like logging or checking conditions when a delete happens.

beginner

When does a BEFORE DELETE trigger execute?

A BEFORE DELETE trigger runs just before a row is deleted from the table. It can be used to check or modify data before the deletion happens.

intermediate

Can a DELETE trigger prevent a row from being deleted?

Yes, a BEFORE DELETE trigger can prevent deletion by signaling an error or condition that stops the delete operation.

intermediate

Write a simple DELETE trigger that logs deleted row IDs into a log table.

CREATE TRIGGER log_delete BEFORE DELETE ON your_table FOR EACH ROW BEGIN INSERT INTO delete_log (deleted_id) VALUES (OLD.id); END;

intermediate

What is the difference between BEFORE DELETE and AFTER DELETE triggers?

BEFORE DELETE triggers run before the row is deleted and can stop the deletion. AFTER DELETE triggers run after the row is deleted and cannot stop the deletion but can perform follow-up actions.

What keyword specifies a trigger that runs before a row is deleted?

A AFTER DELETE

B WHEN DELETE

C ON DELETE

D BEFORE DELETE

BEFORE DELETE triggers run before the deletion happens.

Which of these can a DELETE trigger NOT do?

A Modify the row after deletion

B Prevent deletion in BEFORE DELETE trigger

C Run automatically on delete

D Log deleted rows

After deletion, the row no longer exists, so you cannot modify it.

In MySQL, what does OLD refer to inside a DELETE trigger?

A The new row after deletion

B The row before deletion

C The table name

D The trigger name

OLD refers to the row data before it is deleted.

Which trigger timing cannot stop a delete operation?

A AFTER DELETE

B BEFORE UPDATE

C BEFORE DELETE

D BEFORE INSERT

AFTER DELETE triggers run after deletion and cannot stop it.

What is the correct syntax to create a DELETE trigger in MySQL?

A CREATE TRIGGER trigger_name ON DELETE table_name FOR EACH ROW BEGIN ... END;

B CREATE DELETE TRIGGER trigger_name ON table_name BEGIN ... END;

C CREATE TRIGGER trigger_name BEFORE DELETE ON table_name FOR EACH ROW BEGIN ... END;

D CREATE TRIGGER trigger_name AFTER DELETE table_name BEGIN ... END;

The correct syntax uses CREATE TRIGGER with BEFORE or AFTER DELETE, ON table_name, and FOR EACH ROW.

Explain what a DELETE trigger is and when it runs in MySQL.

Think about what happens before and after a row is removed.

Describe how you can use a DELETE trigger to log deleted rows.

Consider saving the ID of the row before it disappears.