UPDATE trigger with OLD and NEW in SQL - Time & Space Complexity

Time Complexity: UPDATE trigger with OLD and NEW

O(n)

Understanding Time Complexity

When a database updates a row, triggers can run extra code automatically. We want to understand how the time to run this trigger changes as more rows are updated.

How does the trigger's work grow when updating many rows?

Scenario Under Consideration

Analyze the time complexity of the following SQL trigger.

CREATE TRIGGER update_log_trigger
AFTER UPDATE ON employees
FOR EACH ROW
BEGIN
  INSERT INTO employees_log(employee_id, old_salary, new_salary, changed_at)
  VALUES (OLD.id, OLD.salary, NEW.salary, CURRENT_TIMESTAMP);
END;
    

This trigger runs after each employee row is updated. It logs the old and new salary values into a log table.

Identify Repeating Operations

Look for repeated actions inside the trigger.

• Primary operation: The trigger runs once for every updated row.
• How many times: Exactly as many times as there are rows updated.

How Execution Grows With Input

Each updated row causes one insert into the log table, so the work grows directly with the number of rows.

Input Size (n)	Approx. Operations
10	10 inserts into log
100	100 inserts into log
1000	1000 inserts into log

Pattern observation: The work grows in a straight line as the number of updated rows increases.

Final Time Complexity

Time Complexity: O(n)

This means the time to run the trigger grows directly in proportion to how many rows are updated.

Common Mistake

[X] Wrong: "The trigger runs only once no matter how many rows are updated."

[OK] Correct: The trigger is defined FOR EACH ROW, so it runs once per updated row, not just once per update statement.

Interview Connect

Understanding how triggers scale helps you write efficient database code and explain performance in real projects. This skill shows you think about how data size affects work done.

Self-Check

"What if the trigger was defined FOR EACH STATEMENT instead of FOR EACH ROW? How would the time complexity change?"

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