Trigger for audit logging in SQL

Introduction

Triggers help automatically record changes in a database. Audit logging keeps track of who changed what and when.

You want to track changes to important data like user info or transactions.

You need to keep a history of changes for security or compliance reasons.

You want to know who updated or deleted records in your database.

You want to automatically save old data before it changes.

You want to create reports about data changes over time.

Syntax

SQL

CREATE TRIGGER trigger_name
AFTER INSERT OR UPDATE OR DELETE ON table_name
FOR EACH ROW
BEGIN
  -- SQL statements to log changes
END;

Triggers run automatically when data changes happen on the specified table.

You can create triggers for INSERT, UPDATE, or DELETE actions.

Examples

This trigger logs every update on the users table by inserting a record into audit_log.

SQL

CREATE TRIGGER log_user_update
AFTER UPDATE ON users
FOR EACH ROW
BEGIN
  INSERT INTO audit_log(user_id, action, changed_at)
  VALUES (OLD.id, 'update', CURRENT_TIMESTAMP);
END;

This trigger logs every deletion from the orders table.

SQL

CREATE TRIGGER log_delete
AFTER DELETE ON orders
FOR EACH ROW
BEGIN
  INSERT INTO audit_log(order_id, action, changed_at)
  VALUES (OLD.id, 'delete', CURRENT_TIMESTAMP);
END;

Sample Program

This example creates a simple users table and an audit_log table. The trigger log_user_update records every update on users. After updating Alice's name, the audit log shows the change.

SQL

CREATE TABLE audit_log (
  id INT AUTO_INCREMENT PRIMARY KEY,
  user_id INT,
  action VARCHAR(20),
  changed_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);

CREATE TABLE users (
  id INT PRIMARY KEY,
  name VARCHAR(100)
);

CREATE TRIGGER log_user_update
AFTER UPDATE ON users
FOR EACH ROW
BEGIN
  INSERT INTO audit_log(user_id, action, changed_at)
  VALUES (OLD.id, 'update', CURRENT_TIMESTAMP);
END;

INSERT INTO users (id, name) VALUES (1, 'Alice');
UPDATE users SET name = 'Alice Smith' WHERE id = 1;

SELECT * FROM audit_log;

Important Notes

Use OLD to access the previous row data in UPDATE or DELETE triggers.

Use CURRENT_TIMESTAMP to record the exact time of the change.

Test triggers carefully to avoid performance issues on large tables.

Summary

Triggers automatically run on data changes to help with audit logging.

They record who changed data and when by inserting records into an audit table.

Use OLD and NEW keywords to access row data inside triggers.

Practice

1. What is the main purpose of a trigger used for audit logging in SQL?

easy

A. To automatically record changes made to data in a table

B. To speed up query execution on large tables

C. To create backups of the database periodically

D. To enforce user permissions on tables

Solution

1. Step 1: Understand what audit logging means

   Audit logging means keeping a record of changes made to data, such as who changed it and when.

2. Step 2: Identify the role of triggers in audit logging

   Triggers automatically run when data changes and can insert records into an audit table to log these changes.

3. Final Answer:

   To automatically record changes made to data in a table -> Option A

4. Quick Check:

   Audit logging = record data changes automatically [OK]

Hint: Triggers run automatically on data changes for logging [OK]

Common Mistakes:

• Thinking triggers speed up queries
• Confusing audit logging with backups
• Believing triggers manage user permissions

2. Which of the following is the correct syntax to create a trigger that fires after an INSERT on a table named employees?

easy

A. CREATE TRIGGER trg AFTER INSERT ON employees FOR EACH ROW BEGIN ... END;

B. CREATE TRIGGER trg BEFORE UPDATE ON employees FOR EACH ROW BEGIN ... END;

C. CREATE TRIGGER trg AFTER DELETE ON employees FOR EACH ROW BEGIN ... END;

D. CREATE TRIGGER trg ON employees AFTER INSERT BEGIN ... END;

Solution

1. Step 1: Identify correct trigger timing and event

   The question asks for a trigger after INSERT, so it must say AFTER INSERT.

2. Step 2: Check syntax correctness

   CREATE TRIGGER trg AFTER INSERT ON employees FOR EACH ROW BEGIN ... END; uses correct syntax: CREATE TRIGGER name AFTER INSERT ON table FOR EACH ROW BEGIN ... END;

3. Final Answer:

   CREATE TRIGGER trg AFTER INSERT ON employees FOR EACH ROW BEGIN ... END; -> Option A

4. Quick Check:

   Trigger syntax = CREATE TRIGGER ... AFTER INSERT ON ... [OK]

Hint: Use AFTER INSERT ON table FOR EACH ROW for insert triggers [OK]

Common Mistakes:

• Using BEFORE UPDATE instead of AFTER INSERT
• Omitting FOR EACH ROW clause
• Wrong trigger event like DELETE instead of INSERT

3. Given this trigger snippet for audit logging:

CREATE TRIGGER audit_update AFTER UPDATE ON products FOR EACH ROW BEGIN INSERT INTO audit_log (product_id, old_price, new_price) VALUES (OLD.id, OLD.price, NEW.price); END;

What will be inserted into audit_log when a product's price changes from 100 to 120?

medium

A. (product_id: NULL, old_price: 100, new_price: 120)

B. (product_id: product's id, old_price: NULL, new_price: NULL)

C. (product_id: product's id, old_price: 120, new_price: 100)

D. (product_id: product's id, old_price: 100, new_price: 120)

Solution

1. Step 1: Understand OLD and NEW in triggers

   OLD refers to the row before update, NEW refers to the row after update.

2. Step 2: Analyze the insert statement

   The trigger inserts OLD.id, OLD.price, and NEW.price, so product_id is the old id, old_price is 100, new_price is 120.

3. Final Answer:

   (product_id: product's id, old_price: 100, new_price: 120) -> Option D

4. Quick Check:

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

Hint: OLD = old row, NEW = new row in update triggers [OK]

Common Mistakes:

• Mixing OLD and NEW values
• Assuming NULL values for unchanged columns
• Confusing product_id with price values

4. You wrote this trigger to log deletions:

CREATE TRIGGER log_delete AFTER DELETE ON orders FOR EACH ROW BEGIN INSERT INTO audit_orders (order_id, deleted_at) VALUES (NEW.id, NOW()); END;

Why does this trigger cause an error?

medium

A. Missing FOR EACH STATEMENT clause

B. Using NEW keyword in a DELETE trigger causes an error

C. NOW() function is not allowed in triggers

D. Trigger must be BEFORE DELETE, not AFTER DELETE

Solution

1. Step 1: Recall OLD and NEW usage in triggers

   In DELETE triggers, only OLD is available because the row is removed; NEW does not exist.

2. Step 2: Identify the error cause

   The trigger uses NEW.id in a DELETE trigger, which causes an error because NEW is undefined.

3. Final Answer:

   Using NEW keyword in a DELETE trigger causes an error -> Option B

4. Quick Check:

   DELETE triggers use OLD, not NEW [OK]

Hint: Use OLD in DELETE triggers, NEW causes errors [OK]

Common Mistakes:

• Using NEW instead of OLD in DELETE triggers
• Confusing FOR EACH ROW and FOR EACH STATEMENT
• Thinking NOW() is disallowed in triggers

5. You want to create an audit trigger that logs INSERT and UPDATE actions on the customers table into customer_audit. Which approach correctly handles both actions in one trigger?

hard

A. Create two separate triggers: one AFTER INSERT and one AFTER UPDATE

B. Create one AFTER INSERT trigger and ignore UPDATE actions

C. Create one trigger AFTER INSERT OR UPDATE and use conditional logic inside

D. Create a BEFORE DELETE trigger to log INSERT and UPDATE

Solution

1. Step 1: Understand trigger event options

   Some SQL systems allow triggers on multiple events like AFTER INSERT OR UPDATE.

2. Step 2: Use conditional logic inside trigger

   Inside the trigger, check if the event is INSERT or UPDATE to log accordingly.

3. Step 3: Compare options

   Create one trigger AFTER INSERT OR UPDATE and use conditional logic inside correctly combines both events in one trigger with logic; Create two separate triggers: one AFTER INSERT and one AFTER UPDATE uses two triggers, which is valid but not one trigger.

4. Final Answer:

   Create one trigger AFTER INSERT OR UPDATE and use conditional logic inside -> Option C

5. Quick Check:

   One trigger can handle multiple events with conditions [OK]

Hint: Use AFTER INSERT OR UPDATE with IF inside trigger [OK]

Common Mistakes:

• Trying to log UPDATE in an INSERT-only trigger
• Using BEFORE DELETE for INSERT/UPDATE logging
• Not using conditional logic inside multi-event triggers