AFTER UPDATE triggers in MySQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of AFTER UPDATE trigger modifying another table

Given two tables employees and audit_log, and an AFTER UPDATE trigger on employees that inserts a record into audit_log whenever an employee's salary changes, what will be the content of audit_log after this update?

UPDATE employees SET salary = salary + 1000 WHERE id = 3;

MySQL

CREATE TABLE employees (id INT PRIMARY KEY, name VARCHAR(50), salary INT);
CREATE TABLE audit_log (log_id INT AUTO_INCREMENT PRIMARY KEY, emp_id INT, old_salary INT, new_salary INT);

DELIMITER $$
CREATE TRIGGER trg_after_update_salary
AFTER UPDATE ON employees
FOR EACH ROW
BEGIN
  IF OLD.salary <> NEW.salary THEN
    INSERT INTO audit_log(emp_id, old_salary, new_salary) VALUES (OLD.id, OLD.salary, NEW.salary);
  END IF;
END$$
DELIMITER ;

INSERT INTO employees VALUES (3, 'Alice', 5000);

A One row in audit_log: emp_id=3, old_salary=5000, new_salary=6000

B No rows in audit_log

C One row in audit_log: emp_id=3, old_salary=6000, new_salary=6000

D Multiple rows in audit_log with old_salary=5000

💡 Hint

The trigger runs after the update and compares old and new salary values.

The AFTER UPDATE trigger checks if the salary changed. Since the salary increased from 5000 to 6000, it inserts one row recording the old and new salaries.

🧠 Conceptual

intermediate

Understanding WHEN AFTER UPDATE triggers fire

Which of the following statements about AFTER UPDATE triggers in MySQL is true?

A AFTER UPDATE triggers fire before the update operation starts.

B AFTER UPDATE triggers fire after each row is updated.

C AFTER UPDATE triggers can modify the row being updated directly.

D AFTER UPDATE triggers do not have access to OLD and NEW row values.

💡 Hint

Think about when the trigger runs relative to the update and what data it can access.

AFTER UPDATE triggers run after each row is updated and have access to OLD and NEW values. They cannot modify the row being updated but can affect other tables.

📝 Syntax

advanced

Identify the syntax error in AFTER UPDATE trigger

Which option contains a syntax error in this AFTER UPDATE trigger definition?

MySQL

CREATE TRIGGER trg_after_update
AFTER UPDATE ON employees
FOR EACH ROW
BEGIN
  INSERT INTO audit_log(emp_id, old_salary, new_salary) VALUES (OLD.id, OLD.salary, NEW.salary);
END;

A Missing DELIMITER change before and after trigger definition

B Trigger must be BEFORE UPDATE, not AFTER UPDATE

C OLD and NEW cannot be used in AFTER UPDATE triggers

D FOR EACH ROW should be FOR EACH RECORD

💡 Hint

MySQL requires changing the delimiter when defining triggers with multiple statements.

In MySQL, when defining triggers with BEGIN...END blocks, you must change the delimiter temporarily to avoid syntax errors.

🔧 Debug

advanced

Why does this AFTER UPDATE trigger cause an infinite loop?

Consider this trigger on table accounts that updates the last_modified column after any update:

CREATE TRIGGER trg_after_update_accounts AFTER UPDATE ON accounts FOR EACH ROW BEGIN UPDATE accounts SET last_modified = NOW() WHERE id = NEW.id; END;

What is the cause of the infinite loop?

A AFTER UPDATE triggers cannot contain UPDATE statements.

B The trigger uses NOW() which is not allowed in triggers.

C The trigger lacks a WHERE clause to limit updates.

D The trigger updates the same table, causing it to fire repeatedly.

💡 Hint

Think about what happens when the trigger updates the same table it is defined on.

The trigger updates the same row in the table, causing the trigger to fire again and again, creating an infinite loop.

❓ optimization

expert

Optimizing AFTER UPDATE trigger to avoid unnecessary writes

You have an AFTER UPDATE trigger that logs changes only if the status column changes. Which option is the most efficient way to write the trigger to avoid unnecessary inserts?

A Check status change outside the trigger in application code before updating.

B Use BEFORE UPDATE trigger instead of AFTER UPDATE.

C Use IF OLD.status <> NEW.status THEN INSERT ... END IF; inside the trigger.

D INSERT INTO log_table VALUES (...) unconditionally in the trigger.

💡 Hint

Triggers can check old and new values to decide if action is needed.

Using an IF condition inside the trigger to compare OLD and NEW values prevents unnecessary inserts and improves performance.

Practice

1. What is the main purpose of an AFTER UPDATE trigger in MySQL?

easy

A. To execute actions automatically after a row is updated in a table

B. To prevent updates from happening on a table

C. To execute actions before a row is inserted

D. To delete rows after an update

Solution

1. Step 1: Understand trigger timing

   An AFTER UPDATE trigger runs after a row in a table is updated.

2. Step 2: Identify trigger purpose

   Its purpose is to perform extra actions automatically after the update, like logging or updating related data.

3. Final Answer:

   To execute actions automatically after a row is updated in a table -> Option A

4. Quick Check:

   AFTER UPDATE triggers run after update [OK]

Hint: AFTER UPDATE triggers run after row changes [OK]

Common Mistakes:

• Confusing AFTER UPDATE with BEFORE UPDATE
• Thinking triggers prevent updates
• Mixing trigger types like INSERT or DELETE

2. Which of the following is the correct syntax to create an AFTER UPDATE trigger on a table named employees?

easy

A. CREATE TRIGGER trg AFTER INSERT ON employees FOR EACH ROW BEGIN END;

B. CREATE TRIGGER trg AFTER UPDATE ON employees FOR EACH ROW BEGIN END;

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

D. CREATE TRIGGER trg AFTER UPDATE employees FOR EACH ROW BEGIN END;

Solution

1. Step 1: Check trigger timing and event

   The trigger must be AFTER UPDATE on the employees table.

2. Step 2: Verify syntax correctness

   CREATE TRIGGER trg AFTER UPDATE ON employees FOR EACH ROW BEGIN END; uses correct syntax: CREATE TRIGGER trg AFTER UPDATE ON employees FOR EACH ROW BEGIN END;

3. Final Answer:

   CREATE TRIGGER trg AFTER UPDATE ON employees FOR EACH ROW BEGIN END; -> Option B

4. Quick Check:

   Correct syntax includes 'AFTER UPDATE ON table' [OK]

Hint: Syntax: CREATE TRIGGER name AFTER UPDATE ON table [OK]

Common Mistakes:

• Omitting ON keyword before table name
• Using BEFORE instead of AFTER
• Using wrong event like INSERT

3. Given this trigger on table products:

CREATE TRIGGER trg_after_update
AFTER UPDATE ON products
FOR EACH ROW
BEGIN
  INSERT INTO audit_log (product_id, old_price, new_price)
  VALUES (OLD.id, OLD.price, NEW.price);
END;

What happens when a product's price is updated?

medium

A. The audit_log table is deleted

B. The product's price is reverted to the old price

C. The update is blocked and no changes happen

D. A new row is added to audit_log with old and new prices

Solution

1. Step 1: Understand trigger action

   The trigger inserts a row into audit_log after a product update.

2. Step 2: Analyze inserted values

   It records OLD.id, OLD.price, and NEW.price to log price changes.

3. Final Answer:

   A new row is added to audit_log with old and new prices -> Option D

4. Quick Check:

   AFTER UPDATE triggers can log changes [OK]

Hint: AFTER UPDATE can log old and new values [OK]

Common Mistakes:

• Thinking trigger changes original update
• Confusing OLD and NEW values
• Assuming trigger blocks update

4. You wrote this trigger but get an error:

CREATE TRIGGER trg AFTER UPDATE ON orders
FOR EACH ROW
BEGIN
  SET NEW.status = 'processed';
END;

What is the problem?

medium

A. The syntax for SET is incorrect inside triggers

B. The trigger must be BEFORE UPDATE to modify NEW values

C. You cannot modify NEW values in an AFTER UPDATE trigger

D. Triggers cannot use BEGIN...END blocks

Solution

1. Step 1: Understand trigger timing restrictions

   In AFTER UPDATE triggers, NEW values are read-only and cannot be changed.

2. Step 2: Identify correct timing to modify NEW

   To modify NEW values, use BEFORE UPDATE triggers instead.

3. Final Answer:

   You cannot modify NEW values in an AFTER UPDATE trigger -> Option C

4. Quick Check:

   AFTER UPDATE triggers cannot change NEW row [OK]

Hint: Modify NEW only in BEFORE UPDATE triggers [OK]

Common Mistakes:

• Trying to change NEW in AFTER UPDATE triggers
• Confusing BEFORE and AFTER triggers
• Incorrect SET syntax blamed

5. You want to keep a count of how many times each user's email was updated in a table users. Which trigger code correctly updates a email_update_count column after each email change?

hard

A. CREATE TRIGGER trg_email_update
BEFORE UPDATE ON users
FOR EACH ROW
BEGIN
IF OLD.email != NEW.email THEN
SET NEW.email_update_count = OLD.email_update_count + 1;
END IF;
END;

B. CREATE TRIGGER trg_email_update
AFTER UPDATE ON users
FOR EACH ROW
BEGIN
IF OLD.email != NEW.email THEN
UPDATE users SET email_update_count = email_update_count + 1 WHERE id = NEW.id;
END IF;
END;

C. CREATE TRIGGER trg_email_update
AFTER UPDATE ON users
FOR EACH ROW
BEGIN
SET NEW.email_update_count = OLD.email_update_count + 1;
END;

D. CREATE TRIGGER trg_email_update
AFTER UPDATE ON users
FOR EACH ROW
BEGIN
IF OLD.email = NEW.email THEN
UPDATE users SET email_update_count = email_update_count + 1 WHERE id = NEW.id;
END IF;
END;

Solution

1. Step 1: Identify when to modify the count

   The count should increase only if the email changes, so check if OLD.email != NEW.email.

2. Step 2: Choose correct trigger timing to modify NEW

   To change email_update_count in the same row, use a BEFORE UPDATE trigger and set NEW.email_update_count.

3. Step 3: Analyze options

   CREATE TRIGGER trg_email_update
   BEFORE UPDATE ON users
   FOR EACH ROW
   BEGIN
     IF OLD.email != NEW.email THEN
       SET NEW.email_update_count = OLD.email_update_count + 1;
     END IF;
   END;

   correctly uses BEFORE UPDATE, compares emails, and sets NEW value.

   CREATE TRIGGER trg_email_update
   AFTER UPDATE ON users
   FOR EACH ROW
   BEGIN
     IF OLD.email != NEW.email THEN
       UPDATE users SET email_update_count = email_update_count + 1 WHERE id = NEW.id;
     END IF;
   END;

   tries to update the same table in AFTER UPDATE, causing recursion.

   CREATE TRIGGER trg_email_update
   AFTER UPDATE ON users
   FOR EACH ROW
   BEGIN
     SET NEW.email_update_count = OLD.email_update_count + 1;
   END;

   tries to set NEW in AFTER UPDATE (not allowed).

   CREATE TRIGGER trg_email_update
   AFTER UPDATE ON users
   FOR EACH ROW
   BEGIN
     IF OLD.email = NEW.email THEN
       UPDATE users SET email_update_count = email_update_count + 1 WHERE id = NEW.id;
     END IF;
   END;

   updates count when emails are equal (wrong logic).

4. Final Answer:

   BEFORE UPDATE trigger that sets NEW.email_update_count if OLD.email != NEW.email -> Option A

5. Quick Check:

   Modify NEW in BEFORE UPDATE triggers [OK]

Hint: Change NEW only in BEFORE UPDATE, check OLD vs NEW values [OK]

Common Mistakes:

• Trying to modify NEW in AFTER UPDATE triggers
• Updating the same table inside AFTER UPDATE causing recursion
• Wrong condition logic for email change