UPDATE trigger with OLD and NEW in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a trigger that fires before an UPDATE on the employees table.

SQL

CREATE TRIGGER before_employee_update BEFORE UPDATE ON employees FOR EACH ROW BEGIN SET NEW.salary = NEW.salary * [1]; END;

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

A1.1

BOLD.salary

CNEW.salary

D0.9

Attempts:

3 left

💡 Hint

Common Mistakes

Using OLD.salary instead of a number to multiply.

Trying to assign to OLD.salary which is not allowed.

The trigger multiplies the new salary by 1.1 before updating, giving a 10% raise.

2fill in blank

medium

Complete the code to log the old salary value into a salary_changes table inside the trigger.

SQL

CREATE TRIGGER log_salary_change AFTER UPDATE ON employees FOR EACH ROW BEGIN INSERT INTO salary_changes(employee_id, old_salary) VALUES (NEW.id, [1]); END;

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

ANEW.salary

BOLD.id

CNEW.id

DOLD.salary

Attempts:

3 left

💡 Hint

Common Mistakes

Using NEW.salary instead of OLD.salary to log the previous value.

Using NEW.id instead of OLD.id for the employee id.

OLD.salary holds the salary value before the update, which we want to log.

3fill in blank

hard

Fix the error in the trigger code to correctly prevent salary from being set below 3000.

SQL

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

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

AOLD.salary

B3000

Csalary

DNEW.salary

Attempts:

3 left

💡 Hint

Common Mistakes

Using OLD.salary or NEW.salary as the threshold value.

Not using a numeric constant for the minimum salary.

The trigger checks if the new salary is less than 3000 and sets it to 3000 if so.

4fill in blank

hard

Fill both blanks to create a trigger that updates the last_modified column to current timestamp on update.

SQL

CREATE TRIGGER update_timestamp BEFORE UPDATE ON employees FOR EACH ROW BEGIN SET NEW.[1] = [2]; END;

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

Alast_modified

BNOW()

CCURRENT_DATE

Dcreated_at

Attempts:

3 left

💡 Hint

Common Mistakes

Using created_at instead of last_modified.

Using CURRENT_DATE which only gives the date, not time.

The trigger sets the last_modified column to the current timestamp using NOW() before updating.

5fill in blank

hard

Fill all three blanks to create a trigger that prevents changing the employee's department to 'HR' if their salary is below 5000.

SQL

CREATE TRIGGER prevent_hr_update BEFORE UPDATE ON employees FOR EACH ROW BEGIN IF NEW.[1] = 'HR' AND OLD.[2] < [3] THEN SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Salary too low for HR department'; END IF; END;

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

Adepartment

Bsalary

C5000

Did

Attempts:

3 left

💡 Hint

Common Mistakes

Using NEW.salary instead of OLD.salary for the salary check.

Using wrong column names like id instead of department or salary.

The trigger checks if the new department is 'HR' and the old salary is less than 5000, then raises an error.

Practice

1. What do the OLD and NEW keywords represent in an UPDATE trigger?

easy

A. OLD and NEW both represent the data before update

B. OLD is the data after update; NEW is the data before update

C. OLD and NEW both represent the data after update

D. OLD is the data before update; NEW is the data after update

Solution

1. Step 1: Understand trigger timing

   In an UPDATE trigger, OLD holds the row data before the update happens.

2. Step 2: Understand data after update

   NEW holds the row data after the update is applied.

3. Final Answer:

   OLD is the data before update; NEW is the data after update -> Option D

4. Quick Check:

   OLD = before, NEW = after [OK]

Hint: OLD = before update, NEW = after update [OK]

Common Mistakes:

• Confusing OLD and NEW roles
• Thinking both OLD and NEW hold same data
• Assuming OLD is after update

2. Which of the following is the correct syntax to access the new value of a column named salary inside an UPDATE trigger?

easy

A. NEW.salary

B. OLD.salary

C. SELECT salary FROM NEW;

D. UPDATE salary SET NEW;

Solution

1. Step 1: Identify how to reference NEW values in triggers

   Inside an UPDATE trigger, you access new column values using NEW.column_name.

2. Step 2: Match syntax for column salary

   So, the correct way is NEW.salary.

3. Final Answer:

   NEW.salary -> Option A

4. Quick Check:

   Use NEW.column_name to get updated value [OK]

Hint: Use NEW.column_name to get updated column value [OK]

Common Mistakes:

• Using OLD.salary to get new value
• Trying to SELECT from NEW as a table
• Incorrect UPDATE syntax inside trigger

3. Given this trigger code snippet inside an UPDATE trigger:

IF NEW.status = 'active' AND OLD.status = 'inactive' THEN
  INSERT INTO log_table(action) VALUES('Activated');
END IF;

What happens when a row's status changes from 'inactive' to 'active'?

medium

A. No action is taken because OLD and NEW are the same

B. A new row with action 'Activated' is inserted into log_table

C. An error occurs because you cannot compare OLD and NEW

D. The row update is canceled automatically

Solution

1. Step 1: Understand the condition in the trigger

   The condition checks if the new status is 'active' and the old status was 'inactive'.

2. Step 2: Effect of condition being true

   If true, it inserts a row into log_table with action 'Activated'.

3. Final Answer:

   A new row with action 'Activated' is inserted into log_table -> Option B

4. Quick Check:

   Condition true triggers insert [OK]

Hint: Trigger runs code when OLD and NEW differ as condition says [OK]

Common Mistakes:

• Thinking OLD and NEW cannot be compared
• Assuming no insert happens on status change
• Believing update cancels automatically

4. You wrote this trigger code:

CREATE TRIGGER trg_update BEFORE UPDATE ON employees
FOR EACH ROW
BEGIN
  IF NEW.salary < OLD.salary THEN
    SET NEW.salary = OLD.salary;
  END IF;
END;

But when you try to update salary to a lower value, it still changes. What is the problem?

medium

A. The trigger syntax is missing a delimiter or terminator

B. You cannot assign values to NEW in a BEFORE UPDATE trigger

C. The trigger should be AFTER UPDATE, not BEFORE UPDATE

D. The trigger logic is correct; the issue is elsewhere

Solution

1. Step 1: Check trigger syntax completeness

   In many SQL systems, triggers require a delimiter change to define the body properly.

2. Step 2: Missing delimiter causes trigger body to be incomplete

   Without proper delimiters, the trigger may not compile or behave as expected.

3. Final Answer:

   The trigger syntax is missing a delimiter or terminator -> Option A

4. Quick Check:

   Triggers need delimiters to parse body correctly [OK]

Hint: Check delimiters when trigger body has multiple statements [OK]

Common Mistakes:

• Assuming AFTER triggers can modify NEW
• Thinking NEW cannot be assigned in BEFORE triggers
• Ignoring delimiter requirements in SQL triggers

5. You want to create an UPDATE trigger that prevents lowering a product's price. If the new price is less than the old price, the trigger should reset the price to the old value and log the attempt. Which trigger code correctly implements this?

hard

A. CREATE TRIGGER trg_price_check BEFORE UPDATE ON products
FOR EACH ROW
BEGIN
IF OLD.price < NEW.price THEN
SET NEW.price = OLD.price;
INSERT INTO price_log(product_id, old_price, attempted_price) VALUES (OLD.id, OLD.price, NEW.price);
END IF;
END;

B. CREATE TRIGGER trg_price_check AFTER UPDATE ON products
FOR EACH ROW
BEGIN
IF NEW.price < OLD.price THEN
UPDATE products SET price = OLD.price WHERE id = OLD.id;
INSERT INTO price_log(product_id, old_price, attempted_price) VALUES (OLD.id, OLD.price, NEW.price);
END IF;
END;

C. CREATE TRIGGER trg_price_check BEFORE UPDATE ON products
FOR EACH ROW
BEGIN
IF NEW.price < OLD.price THEN
INSERT INTO price_log(product_id, old_price, attempted_price) VALUES (OLD.id, OLD.price, NEW.price);
SET NEW.price = OLD.price;
END IF;
END;

D. CREATE TRIGGER trg_price_check BEFORE UPDATE ON products
FOR EACH ROW
BEGIN
IF NEW.price > OLD.price THEN
SET NEW.price = OLD.price;
INSERT INTO price_log(product_id, old_price, attempted_price) VALUES (OLD.id, OLD.price, NEW.price);
END IF;
END;

Solution

1. Step 1: Identify correct trigger timing and condition

   Use a BEFORE UPDATE trigger to modify NEW values before update. Condition should check if NEW.price is less than OLD.price.

2. Step 2: Correctly reset price and log attempt

   If condition true, set NEW.price = OLD.price to prevent lowering, then insert a log record with OLD and attempted NEW price.

3. Final Answer:

   BEFORE UPDATE trigger that prevents lowering price and logs the attempt -> Option C

4. Quick Check:

   BEFORE UPDATE + NEW.price < OLD.price + reset + log [OK]

Hint: Use BEFORE UPDATE to modify NEW and log changes [OK]

Common Mistakes:

• Using AFTER UPDATE and trying to modify NEW
• Reversing price comparison logic
• Not logging the attempted price change