AFTER trigger execution in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an AFTER trigger in SQL?

An AFTER trigger is a database trigger that runs automatically after a specified data modification event like INSERT, UPDATE, or DELETE has completed successfully.

beginner

When does an AFTER trigger execute in relation to the triggering SQL statement?

An AFTER trigger executes only after the triggering SQL statement has completed successfully.

intermediate

Can an AFTER trigger modify the data that caused it to fire?

No, AFTER triggers cannot modify the data that caused them to fire because the original statement has already completed. They can only perform additional actions like logging or cascading changes.

beginner

Give an example use case for an AFTER trigger.

An AFTER trigger can be used to log changes to a table by inserting a record into an audit table after a row is inserted or updated.

intermediate

What happens if an AFTER trigger fails during execution?

If an AFTER trigger fails, the entire transaction that fired the trigger is rolled back, undoing the original data modification.

When does an AFTER trigger run in SQL?

A Only when a SELECT statement runs

B Before the data modification statement executes

C At the same time as the data modification statement

D After the data modification statement completes successfully

AFTER triggers run only after the triggering data modification statement (INSERT, UPDATE, DELETE) completes successfully.

Which of the following can an AFTER trigger NOT do?

A Log changes to another table

B Send notifications

C Modify the data that caused the trigger to fire

D Enforce complex business rules after data change

AFTER triggers cannot modify the data that caused them to fire because the original statement has already completed.

If an AFTER trigger fails, what happens to the original data change?

A The data change is rolled back

B Only the trigger fails, data change is ignored

C The data change remains committed

D The database shuts down

If an AFTER trigger fails, the entire transaction including the original data change is rolled back.

Which SQL events can an AFTER trigger be defined on?

A INSERT, UPDATE, DELETE

B SELECT only

C CREATE and DROP

D ALTER only

AFTER triggers are commonly defined on data modification events: INSERT, UPDATE, and DELETE.

What is a common use of AFTER triggers?

A To validate data before insertion

B To log audit information after data changes

C To speed up SELECT queries

D To create new tables automatically

AFTER triggers are often used to log audit information or perform actions after data changes.

Explain what an AFTER trigger is and when it executes in a database.

Think about what happens after you save changes in a spreadsheet.

Describe a practical scenario where you would use an AFTER trigger and why.

Consider tracking changes after they happen.

Practice

1. What is the main purpose of an AFTER trigger in SQL?

easy

A. To execute actions only after a successful data change like insert, update, or delete

B. To prevent data changes before they happen

C. To rollback transactions automatically

D. To create new tables after data insertion

Solution

1. Step 1: Understand the timing of AFTER triggers

   AFTER triggers run only after the main data change operation completes successfully.

2. Step 2: Identify the purpose of AFTER triggers

   They automate tasks like logging or updating related data after changes, ensuring consistency.

3. Final Answer:

   To execute actions only after a successful data change like insert, update, or delete -> Option A

4. Quick Check:

   AFTER trigger = post-change action [OK]

Hint: AFTER triggers run only after successful data changes [OK]

Common Mistakes:

• Confusing BEFORE and AFTER triggers
• Thinking AFTER triggers run before data changes
• Assuming AFTER triggers rollback transactions

2. Which of the following is the correct syntax to create an AFTER INSERT trigger in SQL?

easy

A. CREATE TRIGGER trg ON table_name AFTER DELETE BEGIN ... END;

B. CREATE TRIGGER trg AFTER INSERT ON table_name BEGIN ... END;

C. CREATE TRIGGER trg BEFORE INSERT ON table_name BEGIN ... END;

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

Solution

1. Step 1: Recall standard AFTER trigger syntax

   The correct syntax starts with CREATE TRIGGER, followed by trigger name, timing (AFTER), event (INSERT), and table name.

2. Step 2: Check each option

   CREATE TRIGGER trg AFTER INSERT ON table_name BEGIN ... END; matches the correct syntax. Options A, B, and C have wrong order, timing, or event.

3. Final Answer:

   CREATE TRIGGER trg AFTER INSERT ON table_name BEGIN ... END; -> Option B

4. Quick Check:

   CREATE TRIGGER + AFTER + INSERT = correct syntax [OK]

Hint: AFTER triggers syntax: CREATE TRIGGER name AFTER event ON table [OK]

Common Mistakes:

• Mixing BEFORE and AFTER keywords
• Placing AFTER in wrong position
• Using DELETE event instead of INSERT

3. Given this trigger and table:

CREATE TABLE sales (id INT, amount INT);
CREATE TRIGGER trg AFTER INSERT ON sales
BEGIN
  INSERT INTO audit_log VALUES ('insert', NEW.id, NEW.amount);
END;

What happens when you run INSERT INTO sales VALUES (1, 100);?

medium

A. New rows are added to both sales and audit_log

B. A new row is added to audit_log only

C. A new row is added to sales only

D. No rows are added due to trigger error

Solution

1. Step 1: Understand the trigger action

   The AFTER INSERT trigger inserts a row into audit_log using the new sales row data.

2. Step 2: Analyze the insert command effect

   Inserting into sales triggers the AFTER INSERT, so both sales and audit_log get new rows.

3. Final Answer:

   New rows are added to both sales and audit_log -> Option A

4. Quick Check:

   AFTER INSERT triggers run after insert, adding audit_log row [OK]

Hint: AFTER INSERT triggers run after insert, affecting related tables [OK]

Common Mistakes:

• Thinking trigger replaces original insert
• Assuming trigger runs before insert
• Ignoring NEW keyword usage

4. You wrote this AFTER UPDATE trigger:

CREATE TRIGGER trg AFTER UPDATE ON employees
BEGIN
  UPDATE employees SET salary = salary * 1.1 WHERE id = NEW.id;
END;

What problem will occur when you update an employee's salary?

medium

A. Salary will not change

B. Trigger will not run at all

C. Syntax error prevents trigger creation

D. Infinite loop causing stack overflow

Solution

1. Step 1: Analyze trigger logic

   The trigger updates the same table it is triggered on, causing the trigger to fire again.

2. Step 2: Identify the effect of recursive updates

   This causes an infinite loop of updates, eventually causing stack overflow or error.

3. Final Answer:

   Infinite loop causing stack overflow -> Option D

4. Quick Check:

   Trigger updating same table without stop = infinite loop [OK]

Hint: Avoid triggers updating same table without condition to stop recursion [OK]

Common Mistakes:

• Not realizing trigger recursion
• Expecting trigger to run once only
• Ignoring NEW and OLD references

5. You want to log every DELETE from the orders table into orders_log only if the delete succeeds. Which is the best way to do this using an AFTER trigger?

hard

A. Create an AFTER INSERT trigger on orders_log that deletes from orders

B. Create a BEFORE DELETE trigger that inserts into orders_log before deletion

C. Create an AFTER DELETE trigger on orders that inserts deleted row info into orders_log

D. Create a trigger that deletes from orders_log after orders delete

Solution

1. Step 1: Understand timing for logging deletes

   AFTER DELETE triggers run only after the delete succeeds, ensuring accurate logging.

2. Step 2: Choose correct trigger type

   Creating an AFTER DELETE trigger on orders to insert into orders_log captures deleted data reliably.

3. Final Answer:

   Create an AFTER DELETE trigger on orders that inserts deleted row info into orders_log -> Option C

4. Quick Check:

   AFTER DELETE trigger logs only successful deletions [OK]

Hint: Use AFTER DELETE triggers to log only successful deletions [OK]

Common Mistakes:

• Using BEFORE DELETE triggers which may log failed deletes
• Confusing INSERT triggers with DELETE triggers
• Trying to delete from log table after orders delete