UPDATE trigger with OLD and NEW in SQL - Mini Project: Build & Apply

Create an UPDATE Trigger Using OLD and NEW

📖 Scenario: You manage a small library database. You want to keep track of every time a book's price changes. To do this, you will create a trigger that records the old price and the new price whenever an update happens.

🎯 Goal: Build an UPDATE trigger on the books table that uses OLD and NEW to log price changes into a price_changes table.

📋 What You'll Learn

Create a books table with columns id, title, and price.

Create a price_changes table with columns book_id, old_price, and new_price.

Create an UPDATE trigger on books that inserts a record into price_changes whenever the price changes.

Use OLD.price and NEW.price inside the trigger to get the old and new prices.

💡 Why This Matters

🌍 Real World

Tracking changes in data is important for audit logs, history, and debugging in real-world databases.

💼 Career

Database developers and administrators often write triggers to automate tasks and maintain data integrity.

1

Create the books table

Write SQL to create a table called books with columns: id as an integer primary key, title as text, and price as a decimal number.

SQL

-- Create the books table with id, title, and price columns
-- Your code here

Hint

Use CREATE TABLE books and define the columns exactly as specified.

2

Create the price_changes table

Write SQL to create a table called price_changes with columns: book_id as integer, old_price as decimal, and new_price as decimal.

SQL

CREATE TABLE books (
    id INTEGER PRIMARY KEY,
    title TEXT,
    price DECIMAL(10, 2)
);

-- Create the price_changes table with book_id, old_price, and new_price columns
-- Your code here

Hint

Define the price_changes table with the exact columns and types.

3

Start creating the UPDATE trigger

Write SQL to create a trigger named log_price_change that fires AFTER UPDATE on the books table.

SQL

CREATE TABLE books (
    id INTEGER PRIMARY KEY,
    title TEXT,
    price DECIMAL(10, 2)
);

CREATE TABLE price_changes (
    book_id INTEGER,
    old_price DECIMAL(10, 2),
    new_price DECIMAL(10, 2)
);

-- Create an AFTER UPDATE trigger named log_price_change on books
-- Your code here

Hint

Use CREATE TRIGGER with AFTER UPDATE ON books and FOR EACH ROW.

4

Complete the trigger to log price changes using OLD and NEW

Inside the trigger body, write SQL to insert into price_changes the book_id as NEW.id, old_price as OLD.price, and new_price as NEW.price. Only insert if the price has changed (compare OLD.price and NEW.price).

SQL

CREATE TABLE books (
    id INTEGER PRIMARY KEY,
    title TEXT,
    price DECIMAL(10, 2)
);

CREATE TABLE price_changes (
    book_id INTEGER,
    old_price DECIMAL(10, 2),
    new_price DECIMAL(10, 2)
);

CREATE TRIGGER log_price_change
AFTER UPDATE ON books
FOR EACH ROW
BEGIN
    -- Insert into price_changes only if OLD.price is different from NEW.price
    -- Your code here
END;

Hint

Use an IF statement to check if prices differ, then insert the old and new prices using OLD.price and NEW.price.

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