Why DELETE trigger in SQL? - Purpose & Use Cases

The Big Idea

What if deleting one record could magically clean up everything related 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. This takes a lot of time and you might forget some places.

The Problem

Manually deleting related data is slow and risky. You can easily miss some records, causing errors or broken links. It's hard to keep everything clean and consistent by hand.

The Solution

A DELETE trigger automatically runs code when you delete a record. It can remove or update related data instantly, keeping your database clean and consistent without extra work.

Before vs After

✗ Before

DELETE FROM orders WHERE customer_id = 123;
DELETE FROM customers WHERE id = 123;

✓ After

DELETE FROM customers WHERE id = 123; -- DELETE trigger handles orders automatically

What It Enables

DELETE triggers let your database handle complex cleanup tasks automatically, so you never miss important steps.

Real Life Example

In an online store, when a customer account is deleted, a DELETE trigger can automatically remove all their shopping carts and wishlists, keeping the system tidy.

Key Takeaways

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

DELETE triggers automate cleanup tasks on record removal.

This keeps your database consistent and saves time.

Practice

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

easy

A. To insert new rows into a table

B. To update rows before they are deleted

C. To automatically run code when rows are deleted from a table

D. To prevent any data from being deleted

Solution

1. Step 1: Understand what triggers do

   Triggers run automatically in response to certain database events like insert, update, or delete.

2. Step 2: Focus on DELETE triggers

   A DELETE trigger specifically runs when rows are deleted, allowing automatic actions during that event.

3. Final Answer:

   To automatically run code when rows are deleted from a table -> Option C

4. Quick Check:

   DELETE trigger = runs on row deletion [OK]

Hint: DELETE triggers run code automatically on row deletions [OK]

Common Mistakes:

• Confusing DELETE triggers with INSERT triggers
• Thinking DELETE triggers prevent deletion by default
• Assuming DELETE triggers update rows instead

2. Which of the following is the correct syntax to create a trigger that runs before a row is deleted from a table named Employees?

easy

A. CREATE TRIGGER before_delete_employee DELETE ON Employees FOR EACH ROW BEGIN --code END;

B. CREATE TRIGGER before_delete_employee AFTER DELETE ON Employees FOR EACH ROW BEGIN --code END;

C. CREATE TRIGGER before_delete_employee ON Employees BEFORE DELETE FOR EACH ROW BEGIN --code END;

D. CREATE TRIGGER before_delete_employee BEFORE DELETE ON Employees FOR EACH ROW BEGIN --code END;

Solution

1. Step 1: Identify correct trigger timing syntax

   The syntax for a BEFORE DELETE trigger includes 'BEFORE DELETE ON TableName'.

2. Step 2: Check full syntax correctness

   CREATE TRIGGER before_delete_employee BEFORE DELETE ON Employees FOR EACH ROW BEGIN --code END; correctly uses 'CREATE TRIGGER name BEFORE DELETE ON Employees FOR EACH ROW BEGIN ... END;'. Others misuse order or keywords.

3. Final Answer:

   CREATE TRIGGER before_delete_employee BEFORE DELETE ON Employees FOR EACH ROW BEGIN --code END; -> Option D

4. Quick Check:

   BEFORE DELETE ON Table = correct syntax [OK]

Hint: Use BEFORE DELETE ON table for pre-delete triggers [OK]

Common Mistakes:

• Placing BEFORE DELETE after ON table
• Omitting FOR EACH ROW clause
• Using DELETE ON instead of BEFORE DELETE ON

3. Given the following trigger and table data, what will be the output after deleting the row with id = 2 from Orders?

CREATE TRIGGER log_delete AFTER DELETE ON Orders
FOR EACH ROW
BEGIN
  INSERT INTO DeletedOrdersLog(order_id, deleted_at) VALUES (OLD.id, CURRENT_TIMESTAMP);
END;

-- Initial Orders table:
-- id | product
-- 1  | Book
-- 2  | Pen
-- 3  | Notebook

-- DeletedOrdersLog is initially empty.

medium

A. A new row with order_id = 2 is inserted into DeletedOrdersLog

B. The row with id = 2 is restored in Orders

C. No changes happen to DeletedOrdersLog

D. An error occurs because OLD.id is invalid

Solution

1. Step 1: Understand AFTER DELETE trigger behavior

   The trigger runs after a row is deleted from Orders, so OLD.id refers to the deleted row's id.

2. Step 2: Analyze trigger action

   The trigger inserts a new row into DeletedOrdersLog with order_id = OLD.id (2) and current timestamp.

3. Final Answer:

   A new row with order_id = 2 is inserted into DeletedOrdersLog -> Option A

4. Quick Check:

   AFTER DELETE inserts log row with OLD.id [OK]

Hint: AFTER DELETE triggers can log deleted row info using OLD [OK]

Common Mistakes:

• Thinking OLD.id is not accessible in DELETE triggers
• Assuming trigger restores deleted rows
• Believing no action happens after delete

4. Consider this trigger code snippet intended to prevent deletion of rows from Users table if the user is an admin:

CREATE TRIGGER prevent_admin_delete BEFORE DELETE ON Users
FOR EACH ROW
BEGIN
  IF OLD.role = 'admin' THEN
    SIGNAL SQLSTATE '45000' MESSAGE_TEXT = 'Cannot delete admin user';
  END IF;
END;

What is the main issue if this trigger does not prevent admin deletions?

medium

A. The SIGNAL statement syntax is incorrect

B. The condition should check NEW.role instead of OLD.role

C. The trigger does not specify FOR EACH ROW

D. The trigger should be AFTER DELETE, not BEFORE DELETE

Solution

1. Step 1: Check trigger timing and row context

   BEFORE DELETE and FOR EACH ROW are correctly used to check OLD.role before deletion.

2. Step 2: Verify SIGNAL syntax

   In standard SQL, SIGNAL syntax requires correct SQLSTATE and message format. If syntax is wrong, error won't raise and deletion proceeds.

3. Final Answer:

   The SIGNAL statement syntax is incorrect -> Option A

4. Quick Check:

   SIGNAL syntax error prevents trigger from blocking delete [OK]

Hint: Check SIGNAL syntax carefully to block deletes [OK]

Common Mistakes:

• Using AFTER DELETE instead of BEFORE DELETE
• Checking NEW instead of OLD in DELETE triggers
• Omitting FOR EACH ROW clause

5. You want to create a DELETE trigger on a Products table that archives deleted rows into an ArchivedProducts table, but only if the product price is above 100. Which trigger definition correctly implements this logic?

hard

A. CREATE TRIGGER archive_expensive_product BEFORE DELETE ON Products FOR EACH ROW BEGIN IF NEW.price > 100 THEN INSERT INTO ArchivedProducts (id, price) VALUES (NEW.id, NEW.price); END IF; END;

B. CREATE TRIGGER archive_expensive_product AFTER DELETE ON Products FOR EACH ROW BEGIN IF OLD.price > 100 THEN INSERT INTO ArchivedProducts (id, price) VALUES (OLD.id, OLD.price); END IF; END;

C. CREATE TRIGGER archive_expensive_product AFTER DELETE ON Products FOR EACH ROW BEGIN IF NEW.price > 100 THEN INSERT INTO ArchivedProducts (id, price) VALUES (OLD.id, OLD.price); END IF; END;

D. CREATE TRIGGER archive_expensive_product BEFORE DELETE ON Products FOR EACH ROW BEGIN IF NEW.price > 100 THEN INSERT INTO ArchivedProducts (id, price) VALUES (OLD.id, OLD.price); END IF; END;

Solution

1. Step 1: Choose correct timing and row reference

   AFTER DELETE triggers run after deletion; OLD refers to deleted row data. BEFORE DELETE can also work but OLD is preferred after deletion.

2. Step 2: Check condition and insert logic

   Condition must check OLD.price > 100. Insert must use OLD column values to archive deleted row data.

3. Step 3: Validate options

   CREATE TRIGGER archive_expensive_product AFTER DELETE ON Products FOR EACH ROW BEGIN IF OLD.price > 100 THEN INSERT INTO ArchivedProducts (id, price) VALUES (OLD.id, OLD.price); END IF; END; uses AFTER DELETE, OLD.price, and inserts OLD.id, OLD.price correctly. Others use NEW or wrong timing.

4. Final Answer:

   CREATE TRIGGER archive_expensive_product AFTER DELETE ON Products FOR EACH ROW BEGIN IF OLD.price > 100 THEN INSERT INTO ArchivedProducts (id, price) VALUES (OLD.id, OLD.price); END IF; END; -> Option B

5. Quick Check:

   AFTER DELETE + OLD.price condition + insert OLD columns = correct [OK]

Hint: Use AFTER DELETE and OLD to archive deleted rows conditionally [OK]

Common Mistakes:

• Using NEW instead of OLD in DELETE triggers
• Placing condition on NEW.price which is invalid
• Using BEFORE DELETE but referencing OLD incorrectly