AFTER trigger execution in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output after an AFTER INSERT trigger?

Consider a table Orders with columns OrderID and Status. An AFTER INSERT trigger updates the Status to 'Processed' after a new order is inserted.

What will be the Status of the newly inserted order immediately after the insert completes?

SQL

CREATE TABLE Orders (OrderID INT PRIMARY KEY, Status VARCHAR(20));

CREATE TRIGGER trg_AfterInsert
AFTER INSERT ON Orders
FOR EACH ROW
BEGIN
  UPDATE Orders SET Status = 'Processed' WHERE OrderID = NEW.OrderID;
END;

INSERT INTO Orders (OrderID, Status) VALUES (1, 'Pending');

SELECT Status FROM Orders WHERE OrderID = 1;

A 'Processed'

B 'Pending'

C NULL

D Trigger causes error, no output

💡 Hint

Think about when AFTER triggers run in relation to the insert operation.

AFTER INSERT triggers run after the row is inserted. The trigger updates the Status to 'Processed', so the final value is 'Processed'.

🧠 Conceptual

intermediate

When does an AFTER UPDATE trigger execute?

Choose the correct statement about when an AFTER UPDATE trigger runs in a database.

A After the update operation modifies the row

B Before the update operation modifies the row

C Only if the update operation fails

D Before the update operation starts scanning rows

💡 Hint

AFTER triggers run after the triggering event completes.

AFTER UPDATE triggers execute after the row has been updated successfully.

🔧 Debug

advanced

Why does this AFTER DELETE trigger not delete related rows?

Given the following trigger, why does it fail to delete related rows in OrderDetails after deleting from Orders?

SQL

CREATE TRIGGER trg_AfterDelete
AFTER DELETE ON Orders
FOR EACH ROW
BEGIN
  DELETE FROM OrderDetails WHERE OrderID = OLD.OrderID;
END;

DELETE FROM Orders WHERE OrderID = 10;

A The trigger should be BEFORE DELETE to access OLD values

B The trigger syntax is correct and will delete related rows

C The DELETE statement inside the trigger causes a recursion error

D The trigger uses OLD but AFTER triggers cannot access OLD values

💡 Hint

Consider if AFTER DELETE triggers can access OLD values and perform deletes.

AFTER DELETE triggers can access OLD values and perform actions like deleting related rows. The syntax is correct and the trigger will delete related rows.

📝 Syntax

advanced

Identify the syntax error in this AFTER UPDATE trigger

Which option correctly fixes the syntax error in this AFTER UPDATE trigger?

SQL

CREATE TRIGGER trg_AfterUpdate
AFTER UPDATE ON Employees
FOR EACH ROW
BEGIN
  IF NEW.Salary > OLD.Salary THEN
    UPDATE Employees SET Status = 'Promoted' WHERE EmployeeID = NEW.EmployeeID;
  END IF;
END;

A Change AFTER UPDATE to BEFORE UPDATE

B Remove END IF; because it's not needed in triggers

C Replace BEGIN with BEGIN TRANSACTION

D Add THEN after IF condition: IF NEW.Salary > OLD.Salary THEN

💡 Hint

Check the IF statement syntax in SQL triggers.

In SQL triggers, IF statements require THEN after the condition. Missing THEN causes syntax error.

❓ optimization

expert

Optimizing AFTER INSERT trigger to avoid unnecessary updates

An AFTER INSERT trigger updates a column Status to 'Active' for every new row. How can you optimize the trigger to avoid updating rows that already have Status = 'Active'?

A Remove the trigger and update Status manually after inserts

B Change the trigger to BEFORE INSERT and set Status directly

C Add a WHERE clause to update only rows where Status is not 'Active'

D Use a DELETE statement inside the trigger to remove rows with Status 'Active'

💡 Hint

Think about how to limit updates to only necessary rows.

Adding a WHERE clause in the UPDATE statement inside the trigger avoids unnecessary updates and improves performance.

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