Why DELETE triggers in MySQL? - Purpose & Use Cases

The Big Idea

What if deleting one record could instantly clean up everything connected to it without you lifting a finger?

The Scenario

Imagine you have a big list of customers in a spreadsheet. When you delete a customer, you also need to remove all their orders and related info manually from other sheets.

The Problem

Doing this by hand is slow and easy to forget. You might delete the customer but miss some orders, causing confusion and errors later.

The Solution

DELETE triggers automatically run code when you delete a record. They help clean up related data instantly and correctly without extra work.

Before vs After

✗ Before

DELETE FROM customers WHERE id=5;
-- Then manually delete orders for customer 5

✓ After

CREATE TRIGGER after_customer_delete
AFTER DELETE ON customers
FOR EACH ROW
BEGIN
  DELETE FROM orders WHERE customer_id = OLD.id;
END;

What It Enables

It enables automatic, reliable cleanup of related data whenever you delete something important.

Real Life Example

When a user deletes their account on a website, DELETE triggers can remove all their posts, comments, and settings automatically.

Key Takeaways

Manual deletion of related data is slow and error-prone.

DELETE triggers automate cleanup tasks after deletions.

This keeps your database accurate and consistent effortlessly.

Practice

1. What is the main purpose of a DELETE trigger in MySQL?

easy

A. To automatically run code when a row is deleted

B. To insert new rows into a table

C. To update rows before selecting them

D. To create new tables dynamically

Solution

1. Step 1: Understand what triggers do

   Triggers run automatically when certain actions happen on a table, like DELETE.

2. Step 2: Identify the purpose of DELETE triggers

   DELETE triggers specifically run when rows are deleted to perform extra tasks.

3. Final Answer:

   To automatically run code when a row is deleted -> Option A

4. Quick Check:

   DELETE triggers = automatic code on row deletion [OK]

Hint: Triggers run automatically on data changes like DELETE [OK]

Common Mistakes:

• Confusing DELETE triggers with INSERT or UPDATE triggers
• Thinking triggers create or modify tables
• Believing triggers run only manually

2. Which of the following is the correct syntax to create a BEFORE DELETE trigger in MySQL?

easy

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

B. CREATE TRIGGER trg AFTER DELETE ON table_name FOR EACH ROW BEGIN ... END;

C. CREATE TRIGGER trg BEFORE UPDATE ON table_name FOR EACH ROW BEGIN ... END;

D. CREATE TRIGGER trg BEFORE DELETE FROM table_name BEGIN ... END;

Solution

1. Step 1: Recall the correct trigger syntax

   MySQL triggers use CREATE TRIGGER name BEFORE|AFTER event ON table FOR EACH ROW BEGIN ... END;

2. Step 2: Match syntax for BEFORE DELETE

   CREATE TRIGGER trg BEFORE DELETE ON table_name FOR EACH ROW BEGIN ... END; matches this exactly with BEFORE DELETE and correct structure.

3. Final Answer:

   CREATE TRIGGER trg BEFORE DELETE ON table_name FOR EACH ROW BEGIN ... END; -> Option A

4. Quick Check:

   Correct syntax = CREATE TRIGGER trg BEFORE DELETE ON table_name FOR EACH ROW BEGIN ... END; [OK]

Hint: Syntax: CREATE TRIGGER name BEFORE|AFTER DELETE ON table [OK]

Common Mistakes:

• Using AFTER instead of BEFORE when asked
• Wrong event like UPDATE instead of DELETE
• Missing FOR EACH ROW clause

3. Given this trigger and table:

CREATE TABLE users (id INT, name VARCHAR(20));
CREATE TRIGGER log_delete AFTER DELETE ON users FOR EACH ROW BEGIN
INSERT INTO deleted_log VALUES (OLD.id, OLD.name);
END;

What happens when DELETE FROM users WHERE id=1; is executed?

medium

A. The row with id=1 is deleted but nothing is logged

B. The row with id=1 is deleted and its data is inserted into deleted_log

C. The DELETE statement fails with a syntax error

D. The row with id=1 is not deleted because of the trigger

Solution

1. Step 1: Understand AFTER DELETE trigger behavior

   The trigger runs after a row is deleted and inserts OLD row data into deleted_log.

2. Step 2: Analyze the DELETE command effect

   Deleting id=1 removes that row and the trigger logs its data into deleted_log.

3. Final Answer:

   The row with id=1 is deleted and its data is inserted into deleted_log -> Option B

4. Quick Check:

   AFTER DELETE trigger logs deleted row [OK]

Hint: AFTER DELETE triggers run after row removal to log data [OK]

Common Mistakes:

• Thinking trigger prevents deletion
• Assuming no logging happens
• Confusing OLD with NEW in DELETE triggers

4. Identify the error in this trigger definition:

CREATE TRIGGER trg BEFORE DELETE ON users
BEGIN
INSERT INTO deleted_log VALUES (OLD.id, OLD.name);
END;

medium

A. Using NEW in a DELETE trigger is invalid

B. BEFORE DELETE triggers cannot modify data

C. Trigger body must be a single statement without BEGIN...END

D. Missing FOR EACH ROW clause

Solution

1. Step 1: Check trigger syntax requirements

   MySQL requires FOR EACH ROW clause in row-level triggers.

2. Step 2: Identify missing FOR EACH ROW

   The trigger lacks FOR EACH ROW, causing syntax error.

3. Final Answer:

   Missing FOR EACH ROW clause -> Option D

4. Quick Check:

   FOR EACH ROW is mandatory in triggers [OK]

Hint: Always include FOR EACH ROW in row-level triggers [OK]

Common Mistakes:

• Using NEW in DELETE triggers (allowed only OLD)
• Omitting FOR EACH ROW clause
• Incorrect BEGIN...END usage

5. You want to prevent deletion of rows in the orders table if the order status is 'shipped'. Which DELETE trigger logic is correct?

hard

A. CREATE TRIGGER trg BEFORE DELETE ON orders FOR EACH ROW BEGIN IF NEW.status = 'shipped' THEN DELETE FROM orders WHERE id = OLD.id; END IF; END;

B. CREATE TRIGGER trg AFTER DELETE ON orders FOR EACH ROW BEGIN IF NEW.status = 'shipped' THEN ROLLBACK; END IF; END;

C. CREATE TRIGGER trg BEFORE DELETE ON orders FOR EACH ROW BEGIN IF OLD.status = 'shipped' THEN SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Cannot delete shipped orders'; END IF; END;

D. CREATE TRIGGER trg BEFORE DELETE ON orders FOR EACH ROW BEGIN IF OLD.status = 'shipped' THEN UPDATE orders SET status = 'cancelled' WHERE id = OLD.id; END IF; END;

Solution

1. Step 1: Understand how to prevent deletion

   Use BEFORE DELETE trigger and SIGNAL to raise an error and stop deletion.

2. Step 2: Analyze each option's logic

   CREATE TRIGGER trg BEFORE DELETE ON orders FOR EACH ROW BEGIN IF OLD.status = 'shipped' THEN SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Cannot delete shipped orders'; END IF; END; correctly uses BEFORE DELETE, checks OLD.status, and signals error to prevent deletion.

3. Final Answer:

   CREATE TRIGGER trg BEFORE DELETE ON orders FOR EACH ROW BEGIN IF OLD.status = 'shipped' THEN SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Cannot delete shipped orders'; END IF; END; -> Option C

4. Quick Check:

   Use SIGNAL in BEFORE DELETE to block deletion [OK]

Hint: Use SIGNAL in BEFORE DELETE to stop unwanted deletions [OK]

Common Mistakes:

• Using AFTER DELETE to prevent deletion (too late)
• Using NEW in DELETE triggers (only OLD is valid)
• Trying to delete inside a DELETE trigger causing recursion