Trigger for audit logging in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this audit trigger insertion?

Given a trigger that logs inserts into employees table, what will be the content of audit_log after inserting a new employee?

SQL

CREATE TABLE employees (id INT PRIMARY KEY, name VARCHAR(50));
CREATE TABLE audit_log (action VARCHAR(10), emp_id INT, emp_name VARCHAR(50));

CREATE TRIGGER log_employee_insert
AFTER INSERT ON employees
FOR EACH ROW
INSERT INTO audit_log VALUES ('INSERT', NEW.id, NEW.name);

INSERT INTO employees VALUES (1, 'Alice');

SELECT * FROM audit_log;

A

action | emp_id | emp_name
-------+--------+---------
INSERT | 1      | Alice

B

action | emp_id | emp_name
-------+--------+---------
INSERT | NULL   | NULL

C No rows returned

D Syntax error in trigger definition

💡 Hint

Remember that NEW keyword in triggers refers to the inserted row.

The trigger fires after an insert on employees. It inserts a row into audit_log with the action 'INSERT' and the new employee's id and name. So the audit_log table will have one row showing the inserted employee.

❓ Configuration

intermediate

Which trigger definition correctly logs DELETE actions?

Choose the correct SQL trigger that logs deletions from employees into audit_log with action 'DELETE'.

A CREATE TRIGGER log_employee_delete BEFORE DELETE ON employees FOR EACH ROW INSERT INTO audit_log VALUES ('DELETE', NEW.id, NEW.name);

B CREATE TRIGGER log_employee_delete AFTER DELETE ON employees FOR EACH ROW INSERT INTO audit_log VALUES ('DELETE', NEW.id, NEW.name);

C CREATE TRIGGER log_employee_delete AFTER DELETE ON employees FOR EACH ROW INSERT INTO audit_log VALUES ('DELETE', OLD.id, OLD.name);

D CREATE TRIGGER log_employee_delete BEFORE DELETE ON employees FOR EACH ROW INSERT INTO audit_log VALUES ('DELETE', OLD.id, OLD.name);

💡 Hint

For DELETE triggers, OLD holds the row being deleted.

After DELETE triggers run after the row is deleted, and OLD contains the deleted row's data. Using NEW in DELETE triggers is invalid. The correct syntax uses AFTER DELETE and OLD.

❓ Troubleshoot

advanced

Why does this audit trigger fail to log updates?

Given this trigger, why does it fail to log the updated data in audit_log after updating employees?

SQL

CREATE TRIGGER log_employee_update
AFTER UPDATE ON employees
FOR EACH ROW
INSERT INTO audit_log VALUES ('UPDATE', OLD.id, OLD.name);

A The trigger is AFTER UPDATE but should be BEFORE UPDATE.

B The trigger uses OLD values instead of NEW values for updated data.

C The trigger syntax is invalid because it lacks BEGIN...END block.

D The audit_log table does not exist.

💡 Hint

Consider which values reflect the updated data after an update.

After an update, OLD holds the old data before update, NEW holds the new data. To log the updated values, the trigger should insert NEW values, not OLD. Using OLD logs old data, which might be confusing or appear as no change.

🔀 Workflow

advanced

Order the steps to create an audit logging trigger for INSERT and UPDATE

Put these steps in the correct order to create a trigger that logs both INSERT and UPDATE actions on employees.

A 3,2,1,4

B 2,1,3,4

C 1,3,2,4

D 1,2,3,4

💡 Hint

Think about what you need before creating triggers and testing.

First create the audit_log table to store logs. Then write the trigger function that inserts logs. Next create the trigger that calls this function on insert or update. Finally, test the setup.

✅ Best Practice

expert

Which approach best ensures audit logging does not block main transactions?

In a high-traffic database, which method best prevents audit logging triggers from slowing down main data changes?

A Use asynchronous logging by writing audit entries to a queue table and processing them later

B Write audit logs synchronously inside the trigger using INSERT statements

C Disable triggers during peak hours to improve performance

D Log audit data only once per day using a scheduled batch job

💡 Hint

Consider how to avoid slowing down the main transaction.

Writing audit logs synchronously inside triggers can slow down transactions. Disabling triggers or batching once a day risks losing audit detail or timeliness. Using asynchronous logging with a queue table lets main transactions finish quickly, and audit logs are processed separately.

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