Why UPDATE trigger with OLD and NEW in SQL? - Purpose & Use Cases

The Big Idea

What if your database could remember every change for you, perfectly and instantly?

The Scenario

Imagine you have a big notebook where you write down all changes made to your friends' contact details by hand. Every time a phone number changes, you try to remember the old one and write the new one next to it.

The Problem

This manual way is slow and confusing. You might forget the old number, mix up entries, or lose track of who changed what and when. It's hard to keep everything accurate and up-to-date.

The Solution

An UPDATE trigger with OLD and NEW values automatically watches for changes in your data. It remembers the old information and compares it with the new, so you can track updates easily and react to them without mistakes.

Before vs After

✗ Before

Manually check old value, then update and log changes in separate steps.

✓ After

CREATE TRIGGER trg_update BEFORE UPDATE ON contacts FOR EACH ROW BEGIN -- OLD and NEW let you compare and act on changes; END;

What It Enables

This lets your database automatically track and respond to data changes, keeping your information accurate and reliable without extra effort.

Real Life Example

When a customer updates their address, the trigger can save the old address for records and notify the shipping department about the change instantly.

Key Takeaways

Manual tracking of data changes is slow and error-prone.

UPDATE triggers use OLD and NEW to automatically detect and handle changes.

This keeps data accurate and helps automate important follow-up actions.

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