COMMIT and ROLLBACK behavior in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the COMMIT command do in SQL?

COMMIT saves all changes made in the current transaction permanently to the database.

beginner

What happens when you use ROLLBACK in SQL?

ROLLBACK undoes all changes made in the current transaction, restoring the database to its previous state.

beginner

When should you use COMMIT in a transaction?

Use COMMIT after you have completed all your changes and want to save them permanently.

intermediate

Can you undo a COMMIT with ROLLBACK?

No, once you COMMIT, changes are permanent and cannot be undone by ROLLBACK.

beginner

What is a transaction in SQL?

A transaction is a group of one or more SQL statements executed as a single unit, which can be committed or rolled back together.

What does the ROLLBACK command do?

A Undoes changes in the current transaction

B Saves changes permanently

C Starts a new transaction

D Deletes the database

ROLLBACK undoes all changes made in the current transaction.

When are changes saved permanently in a database?

A After ROLLBACK

B When the database is restarted

C Before starting a transaction

D After COMMIT

COMMIT saves changes permanently.

Can you undo a COMMIT with ROLLBACK?

A Yes, always

B Only if the database supports it

C No, COMMIT is permanent

D Only within 5 minutes

Once committed, changes cannot be undone by ROLLBACK.

What is a transaction?

A A group of SQL statements executed as one unit

B A backup of the database

C A single SQL statement

D A database user

A transaction groups SQL statements to be committed or rolled back together.

Which command permanently saves changes made in a transaction?

A SAVEPOINT

B COMMIT

C ROLLBACK

D BEGIN

COMMIT saves all changes permanently.

Explain the difference between COMMIT and ROLLBACK in SQL.

Think about saving vs undoing changes.

Describe what a transaction is and how COMMIT and ROLLBACK affect it.

Consider a transaction like a batch of changes.

Practice

1. What does the COMMIT command do in a database transaction?

easy

A. It locks the database to prevent changes.

B. It undoes all changes made during the transaction.

C. It starts a new transaction.

D. It saves all changes made during the transaction permanently.

Solution

1. Step 1: Understand the purpose of COMMIT

   The COMMIT command finalizes all changes made in the current transaction and saves them permanently in the database.

2. Step 2: Compare with other options

   ROLLBACK undoes changes, starting a transaction is done by BEGIN or START TRANSACTION, and locking is a different operation.

3. Final Answer:

   It saves all changes made during the transaction permanently. -> Option D

4. Quick Check:

   COMMIT = Save changes permanently [OK]

Hint: COMMIT means save changes permanently [OK]

Common Mistakes:

• Confusing COMMIT with ROLLBACK
• Thinking COMMIT starts a transaction
• Assuming COMMIT locks the database

2. Which of the following is the correct syntax to undo changes made in the current transaction?

easy

A. COMMIT;

B. ROLLBACK;

C. UNDO;

D. SAVE;

Solution

1. Step 1: Identify the command to undo changes

   The ROLLBACK command is used to undo all changes made since the last COMMIT.

2. Step 2: Check syntax correctness

   ROLLBACK; is the correct syntax. COMMIT; saves changes, UNDO; and SAVE; are not valid SQL commands for this purpose.

3. Final Answer:

   ROLLBACK; -> Option B

4. Quick Check:

   ROLLBACK = Undo changes [OK]

Hint: ROLLBACK undoes changes, COMMIT saves them [OK]

Common Mistakes:

• Using COMMIT to undo changes
• Assuming UNDO is a valid SQL command
• Confusing SAVE with COMMIT

3. Consider this sequence of SQL commands:

BEGIN TRANSACTION;
INSERT INTO users (id, name) VALUES (1, 'Alice');
ROLLBACK;
SELECT * FROM users WHERE id = 1;

What will the SELECT query return?

medium

A. No rows

B. A row with id=1 and name='Alice'

C. Syntax error

D. All rows in the users table

Solution

1. Step 1: Analyze the transaction commands

   The INSERT adds a row with id=1 and name='Alice', but then ROLLBACK undoes this change.

2. Step 2: Understand the effect of ROLLBACK on SELECT

   Since the insert was rolled back, the row does not exist. The SELECT query will find no rows with id=1.

3. Final Answer:

   No rows -> Option A

4. Quick Check:

   ROLLBACK removes inserted rows [OK]

Hint: ROLLBACK removes changes before SELECT runs [OK]

Common Mistakes:

• Assuming inserted row remains after ROLLBACK
• Thinking ROLLBACK affects only future queries
• Confusing ROLLBACK with COMMIT

4. You run this SQL code:

BEGIN TRANSACTION;
UPDATE accounts SET balance = balance - 100 WHERE id = 5;
COMMIT;
ROLLBACK;

What is the effect of the final ROLLBACK?

medium

A. It undoes the UPDATE and restores the original balance.

B. It causes a syntax error because ROLLBACK cannot follow COMMIT.

C. It has no effect because changes were already committed.

D. It commits the changes again.

Solution

1. Step 1: Understand COMMIT effect

   COMMIT saves all changes permanently, so the UPDATE is finalized.

2. Step 2: Analyze ROLLBACK after COMMIT

   ROLLBACK after COMMIT does nothing because there is no active transaction to undo.

3. Final Answer:

   It has no effect because changes were already committed. -> Option C

4. Quick Check:

   ROLLBACK after COMMIT = no effect [OK]

Hint: ROLLBACK only undoes uncommitted changes [OK]

Common Mistakes:

• Thinking ROLLBACK undoes committed changes
• Assuming ROLLBACK causes error after COMMIT
• Confusing order of commands

5. You want to insert multiple rows into a table but only save them if all inserts succeed. Which sequence correctly uses COMMIT and ROLLBACK to achieve this?

1. BEGIN TRANSACTION;
2. INSERT INTO orders VALUES (101, 'Book');
3. INSERT INTO orders VALUES (102, 'Pen');
4. -- If both inserts succeed, then ___;
5. -- If any insert fails, then ___;

hard

A. 4: COMMIT; 5: ROLLBACK;

B. 4: ROLLBACK; 5: COMMIT;

C. 4: COMMIT; 5: COMMIT;

D. 4: ROLLBACK; 5: ROLLBACK;

Solution

1. Step 1: Understand transaction control

   To save changes only if all inserts succeed, you COMMIT after success and ROLLBACK if any fail.

2. Step 2: Match commands to conditions

   Line 4 should be COMMIT to save changes; line 5 should be ROLLBACK to undo partial changes.

3. Final Answer:

   4: COMMIT; 5: ROLLBACK; -> Option A

4. Quick Check:

   COMMIT on success, ROLLBACK on failure [OK]

Hint: COMMIT saves all; ROLLBACK undoes all [OK]

Common Mistakes:

• Swapping COMMIT and ROLLBACK roles
• Committing even if inserts fail
• Rolling back after successful inserts