BEFORE UPDATE triggers in MySQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a BEFORE UPDATE trigger that sets the new value of a column.

MySQL

CREATE TRIGGER update_salary BEFORE UPDATE ON employees FOR EACH ROW SET NEW.salary = NEW.salary [1] 1000;

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

A-

B*

C+

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' will decrease the salary instead of increasing it.

Using '*' or '/' will multiply or divide the salary, which is not intended here.

The trigger adds 1000 to the new salary before updating the row.

2fill in blank

medium

Complete the code to prevent updating the salary if the new salary is less than the old salary.

MySQL

CREATE TRIGGER check_salary BEFORE UPDATE ON employees FOR EACH ROW BEGIN IF NEW.salary [1] OLD.salary THEN SET NEW.salary = OLD.salary; END IF; END;

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

A=

B>

C>=

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' would check if the new salary is greater, which is not the goal.

Using '=' or '>=' will not correctly prevent salary decrease.

The trigger checks if the new salary is less than the old salary and resets it to the old salary to prevent decrease.

3fill in blank

hard

Fix the error in the trigger code to correctly assign a new value to a column in a BEFORE UPDATE trigger.

MySQL

CREATE TRIGGER update_bonus BEFORE UPDATE ON employees FOR EACH ROW SET [1].bonus = NEW.bonus + 500;

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

ANEW

Bbonus

Cemployees

DOLD

Attempts:

3 left

💡 Hint

Common Mistakes

Using OLD instead of NEW causes errors because OLD is read-only.

Using the table name or column name alone is invalid syntax.

In BEFORE UPDATE triggers, you assign new values using the NEW keyword, not OLD or table names.

4fill in blank

hard

Fill both blanks to create a BEFORE UPDATE trigger that prevents the 'status' column from being changed to 'inactive'.

MySQL

CREATE TRIGGER prevent_inactive BEFORE UPDATE ON users FOR EACH ROW BEGIN IF NEW.status [1] 'inactive' THEN SET NEW.status = [2].status; END IF; END;

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

A=

B!=

COLD

DNEW

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' would check for inequality, which is not correct here.

Using NEW.status to reset the value causes no change.

The trigger checks if the new status is 'inactive' and resets it to the old status to prevent the change.

5fill in blank

hard

Fill all three blanks to create a BEFORE UPDATE trigger that increases 'points' by 10 only if the new 'level' is greater than the old 'level'.

MySQL

CREATE TRIGGER level_up BEFORE UPDATE ON players FOR EACH ROW BEGIN IF NEW.level [1] OLD.level THEN SET NEW.points = NEW.points [2] [3]; END IF; END;

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

A>

B+

C10

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' would check the wrong condition.

Using '-' instead of '+' would subtract points.

The trigger checks if the new level is greater than the old level, then adds 10 points to the new points value.

Practice

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

easy

A. To execute code before a row is updated, allowing data validation or modification.

B. To execute code after a row is updated, for logging changes.

C. To prevent any updates on a table permanently.

D. To create a backup of the table before updating.

Solution

1. Step 1: Understand trigger timing

   BEFORE UPDATE triggers run before the data is changed in the table.

2. Step 2: Purpose of the trigger

   This timing allows checking or modifying the new data before it is saved.

3. Final Answer:

   To execute code before a row is updated, allowing data validation or modification. -> Option A

4. Quick Check:

   BEFORE UPDATE triggers run before update [OK]

Hint: BEFORE UPDATE triggers run before saving changes [OK]

Common Mistakes:

• Confusing BEFORE UPDATE with AFTER UPDATE triggers
• Thinking triggers prevent updates entirely
• Assuming triggers create backups automatically

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

easy

A. CREATE TRIGGER before_update_employees BEFORE UPDATE ON employees BEGIN SET NEW.salary = NEW.salary * 1.1; END;

B. CREATE TRIGGER before_update_employees AFTER UPDATE ON employees FOR EACH ROW BEGIN SET NEW.salary = NEW.salary * 1.1; END;

C. CREATE TRIGGER before_update_employees BEFORE UPDATE employees FOR EACH ROW BEGIN SET NEW.salary = NEW.salary * 1.1; END;

D. CREATE TRIGGER before_update_employees BEFORE UPDATE ON employees FOR EACH ROW BEGIN SET NEW.salary = NEW.salary * 1.1; END;

Solution

1. Step 1: Check trigger timing and table

   The syntax requires BEFORE UPDATE ON employees to specify timing and table.

2. Step 2: Confirm trigger body and FOR EACH ROW

   The trigger must include FOR EACH ROW and a BEGIN...END block for multiple statements.

3. Final Answer:

   CREATE TRIGGER before_update_employees BEFORE UPDATE ON employees FOR EACH ROW BEGIN SET NEW.salary = NEW.salary * 1.1; END; -> Option D

4. Quick Check:

   Correct syntax includes BEFORE UPDATE ON and FOR EACH ROW [OK]

Hint: Use BEFORE UPDATE ON table FOR EACH ROW syntax [OK]

Common Mistakes:

• Using AFTER UPDATE instead of BEFORE UPDATE
• Omitting ON keyword before table name
• Missing FOR EACH ROW clause

3. Given this trigger and table:

CREATE TABLE products (id INT, price DECIMAL(5,2));
CREATE TRIGGER price_check BEFORE UPDATE ON products FOR EACH ROW BEGIN
  IF NEW.price < 0 THEN SET NEW.price = 0; END IF;
END;

What happens if you run:

UPDATE products SET price = -10 WHERE id = 1;

medium

A. The price is set to -10 because triggers don't change values.

B. The update fails with a syntax error.

C. The price is set to 0 because the trigger changes negative prices to zero.

D. The price remains unchanged because the trigger runs after update.

Solution

1. Step 1: Understand trigger logic

   The trigger checks if the new price is less than 0, then sets it to 0.

2. Step 2: Apply update and trigger effect

   Updating price to -10 triggers the BEFORE UPDATE trigger, which changes it to 0 before saving.

3. Final Answer:

   The price is set to 0 because the trigger changes negative prices to zero. -> Option C

4. Quick Check:

   BEFORE UPDATE can modify NEW values [OK]

Hint: BEFORE UPDATE can fix invalid data before saving [OK]

Common Mistakes:

• Thinking triggers run after update
• Assuming negative price is saved as is
• Expecting syntax error from trigger

4. You wrote this trigger:

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

But when updating rows, the status does not change. What is the likely problem?

medium

A. The trigger syntax is missing a delimiter change, causing a syntax error.

B. You cannot assign values to NEW in BEFORE UPDATE triggers.

C. The trigger must be AFTER UPDATE to modify data.

D. The table 'orders' does not have a 'status' column.

Solution

1. Step 1: Check trigger syntax requirements

   MySQL requires changing the delimiter when creating triggers with BEGIN...END blocks.

2. Step 2: Understand effect of missing delimiter

   Without changing delimiter, the trigger creation fails silently or is incomplete, so it doesn't run.

3. Final Answer:

   The trigger syntax is missing a delimiter change, causing a syntax error. -> Option A

4. Quick Check:

   Use DELIMITER to create triggers with BEGIN...END [OK]

Hint: Remember to change delimiter when creating triggers with BEGIN...END [OK]

Common Mistakes:

• Not changing delimiter before CREATE TRIGGER
• Thinking NEW cannot be assigned in BEFORE UPDATE
• Assuming AFTER UPDATE is needed to modify NEW

5. You want to prevent users from lowering their account_balance in the users table. Which BEFORE UPDATE trigger code correctly enforces this rule?

hard

A. CREATE TRIGGER check_balance BEFORE UPDATE ON users FOR EACH ROW BEGIN IF NEW.account_balance < 0 THEN SET NEW.account_balance = 0; END IF; END;

B. CREATE TRIGGER check_balance BEFORE UPDATE ON users FOR EACH ROW BEGIN IF NEW.account_balance < OLD.account_balance THEN SET NEW.account_balance = OLD.account_balance; END IF; END;

C. CREATE TRIGGER check_balance AFTER UPDATE ON users FOR EACH ROW BEGIN IF NEW.account_balance < OLD.account_balance THEN SET NEW.account_balance = OLD.account_balance; END IF; END;

D. CREATE TRIGGER check_balance BEFORE UPDATE ON users FOR EACH ROW BEGIN IF NEW.account_balance > OLD.account_balance THEN SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Cannot increase balance'; END IF; END;

Solution

1. Step 1: Understand the requirement

   The trigger must prevent lowering the account_balance by resetting it to the old value if decreased.

2. Step 2: Analyze options

   CREATE TRIGGER check_balance BEFORE UPDATE ON users FOR EACH ROW BEGIN IF NEW.account_balance < OLD.account_balance THEN SET NEW.account_balance = OLD.account_balance; END IF; END; resets NEW.account_balance to OLD.account_balance if it is less, correctly preventing decrease.

3. Step 3: Check other options

   The code that signals an error prevents increases instead of decreases. AFTER UPDATE triggers cannot modify NEW values. The code that checks if balance is negative only prevents negative values but not decreases to lower positive values.

4. Final Answer:

   CREATE TRIGGER check_balance BEFORE UPDATE ON users FOR EACH ROW BEGIN IF NEW.account_balance < OLD.account_balance THEN SET NEW.account_balance = OLD.account_balance; END IF; END; -> Option B

5. Quick Check:

   Use BEFORE UPDATE and compare NEW vs OLD to prevent decrease [OK]

Hint: Compare NEW and OLD values in BEFORE UPDATE to enforce rules [OK]

Common Mistakes:

• Using AFTER UPDATE to modify NEW values
• Signaling error for wrong condition
• Only checking for negative values, not decreases