DELETE triggers in MySQL

Introduction

A DELETE trigger automatically runs a set of actions when a row is removed from a table. It helps keep data consistent or perform extra tasks without manual work.

You want to log every time a record is deleted from a table.

You need to update related tables when a row is removed.

You want to prevent deletion under certain conditions.

You want to archive deleted data automatically.

You want to maintain audit trails for data changes.

Syntax

MySQL

CREATE TRIGGER trigger_name
BEFORE|AFTER DELETE
ON table_name
FOR EACH ROW
BEGIN
  -- SQL statements here
END;

Triggers can run BEFORE or AFTER the DELETE action.

Use OLD.column_name to access the deleted row's data inside the trigger.

Examples

This trigger runs before a row is deleted from the employees table.

MySQL

CREATE TRIGGER before_delete_employee
BEFORE DELETE ON employees
FOR EACH ROW
BEGIN
  -- actions before deleting an employee
END;

This trigger logs deleted orders after they are removed.

MySQL

CREATE TRIGGER after_delete_order
AFTER DELETE ON orders
FOR EACH ROW
BEGIN
  INSERT INTO deleted_orders_log(order_id, deleted_at) VALUES (OLD.id, NOW());
END;

Sample Program

This example creates a trigger that logs deleted employees into another table with the deletion time.

MySQL

CREATE TABLE employees (
  id INT PRIMARY KEY,
  name VARCHAR(50)
);

CREATE TABLE employee_deletions (
  emp_id INT,
  deleted_on DATETIME
);

DELIMITER $$
CREATE TRIGGER after_employee_delete
AFTER DELETE ON employees
FOR EACH ROW
BEGIN
  INSERT INTO employee_deletions(emp_id, deleted_on) VALUES (OLD.id, NOW());
END$$
DELIMITER ;

INSERT INTO employees VALUES (1, 'Alice'), (2, 'Bob');

DELETE FROM employees WHERE id = 1;

SELECT * FROM employee_deletions;

Important Notes

Remember to change the DELIMITER when creating triggers in MySQL to avoid syntax errors.

Use OLD to refer to the row being deleted inside the trigger.

Triggers cannot call COMMIT or ROLLBACK statements.

Summary

DELETE triggers run automatically when rows are deleted.

They can run BEFORE or AFTER the deletion.

Use them to log, prevent, or react to deletions.

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