BEFORE UPDATE triggers in MySQL

Introduction

A BEFORE UPDATE trigger lets you run some actions automatically just before a row in a table is changed. It helps you check or change data before saving.

You want to check if new data meets rules before updating a record.

You want to automatically change or fix data before it is saved.

You want to keep a log of old and new values before changes happen.

You want to prevent certain updates based on conditions.

You want to update related data automatically when a record changes.

Syntax

MySQL

CREATE TRIGGER trigger_name
BEFORE UPDATE ON table_name
FOR EACH ROW
BEGIN
  -- statements to run before update
END;

The trigger runs once for each row that is updated.

Use NEW.column_name to access the new value being set.

Examples

This trigger sets age to 0 if someone tries to update it to a negative number.

MySQL

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

This trigger updates the 'updated_at' column to the current time before any update.

MySQL

CREATE TRIGGER update_timestamp_before_update
BEFORE UPDATE ON orders
FOR EACH ROW
BEGIN
  SET NEW.updated_at = NOW();
END;

Sample Program

This example creates a products table and a BEFORE UPDATE trigger that sets price to 0 if a negative price is given and updates the last_modified timestamp. Then it tries to update price to -5, but the trigger changes it to 0.

MySQL

CREATE TABLE products (
  id INT PRIMARY KEY,
  name VARCHAR(50),
  price DECIMAL(10,2),
  last_modified TIMESTAMP
);

DELIMITER $$
CREATE TRIGGER before_product_update
BEFORE UPDATE ON products
FOR EACH ROW
BEGIN
  IF NEW.price < 0 THEN
    SET NEW.price = 0;
  END IF;
  SET NEW.last_modified = NOW();
END$$
DELIMITER ;

INSERT INTO products VALUES (1, 'Pen', 1.50, NOW());

UPDATE products SET price = -5 WHERE id = 1;

SELECT * FROM products WHERE id = 1;

Important Notes

Remember to use DELIMITER to define triggers in MySQL because triggers contain multiple statements.

BEFORE UPDATE triggers can modify the NEW values before they are saved.

You cannot change OLD values in triggers; they represent the current stored data.

Summary

BEFORE UPDATE triggers run before a row is updated in a table.

They let you check or change data before saving.

Use NEW to access or modify the new data values.

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