DELETE triggers in MySQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a DELETE trigger that activates before a row is deleted.

MySQL

CREATE TRIGGER before_delete_example BEFORE DELETE ON employees FOR EACH ROW BEGIN [1]; END;

Drag options to blanks, or click blank then click option'

ASET @old_salary = OLD.salary

BINSERT INTO log_table VALUES ('deleted')

CUPDATE employees SET salary = 0

DSELECT * FROM employees

Attempts:

3 left

💡 Hint

Common Mistakes

Using NEW keyword in DELETE triggers (NEW is for INSERT/UPDATE).

Trying to SELECT inside a trigger without INTO clause.

The trigger uses OLD to access the row being deleted. 'SET @old_salary = OLD.salary' stores the salary before deletion.

2fill in blank

medium

Complete the code to log the deleted employee's ID into a log table.

MySQL

CREATE TRIGGER log_delete_employee AFTER DELETE ON employees FOR EACH ROW BEGIN INSERT INTO delete_log(employee_id) VALUES([1]); END;

Drag options to blanks, or click blank then click option'

ANEW.id

Bdeleted_id

Cid

DOLD.id

Attempts:

3 left

💡 Hint

Common Mistakes

Using NEW.id which is invalid in DELETE triggers.

Using column name without OLD or NEW prefix.

In DELETE triggers, OLD refers to the row being deleted. So OLD.id gives the employee's ID before deletion.

3fill in blank

hard

Fix the error in the trigger code to prevent syntax issues.

MySQL

CREATE TRIGGER delete_check BEFORE DELETE ON orders FOR EACH ROW BEGIN IF [1] THEN SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Cannot delete shipped order'; END IF; END;

Drag options to blanks, or click blank then click option'

ANEW.status = 'shipped'

BOLD.status = 'shipped'

Cstatus = 'shipped'

DOLD.status == 'shipped'

Attempts:

3 left

💡 Hint

Common Mistakes

Using NEW in DELETE triggers.

Using '==' instead of '=' for comparison.

Not prefixing column with OLD.

Use OLD.status to check the status of the row being deleted. Use single '=' for comparison in SQL.

4fill in blank

hard

Fill both blanks to create a trigger that prevents deletion if quantity is greater than zero.

MySQL

CREATE TRIGGER prevent_delete BEFORE DELETE ON inventory FOR EACH ROW BEGIN IF [1] [2] 0 THEN SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Cannot delete item with stock'; END IF; END;

Drag options to blanks, or click blank then click option'

AOLD.quantity

BNEW.quantity

C>

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using NEW in DELETE triggers.

Using '<=' instead of '>' causing wrong logic.

Use OLD.quantity to check the current row's quantity. Use '>' to check if quantity is greater than zero.

5fill in blank

hard

Fill all three blanks to create a DELETE trigger that archives deleted rows.

MySQL

CREATE TRIGGER archive_delete AFTER DELETE ON products FOR EACH ROW BEGIN INSERT INTO archive (product_id, name, deleted_at) VALUES ([1], [2], [3]); END;

Drag options to blanks, or click blank then click option'

AOLD.product_id

BOLD.name

CNOW()

DNEW.name

Attempts:

3 left

💡 Hint

Common Mistakes

Using NEW in DELETE triggers.

Forgetting to use NOW() for timestamp.

Use OLD to access deleted row's columns. Use NOW() to record deletion time.

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