Why transactions ensure data integrity in MySQL - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start a transaction in MySQL.

MySQL

START [1];

Drag options to blanks, or click blank then click option'

ASAVEPOINT

BCOMMIT

CROLLBACK

DTRANSACTION

Attempts:

3 left

💡 Hint

Common Mistakes

Using COMMIT or ROLLBACK to start a transaction.

Using SAVEPOINT to start a transaction.

The START TRANSACTION command begins a new transaction block in MySQL.

2fill in blank

medium

Complete the code to save changes permanently in a transaction.

MySQL

[1];

Drag options to blanks, or click blank then click option'

ASTART TRANSACTION

BROLLBACK

CCOMMIT

DSAVEPOINT

Attempts:

3 left

💡 Hint

Common Mistakes

Using START TRANSACTION to save changes.

Using ROLLBACK instead of COMMIT.

The COMMIT command saves all changes made during the transaction permanently to the database.

3fill in blank

hard

Fix the error in the code to undo changes in a transaction.

MySQL

[1];

Drag options to blanks, or click blank then click option'

ACOMMIT

BROLLBACK

CSTART TRANSACTION

DSAVEPOINT

Attempts:

3 left

💡 Hint

Common Mistakes

Using COMMIT to undo changes.

Using START TRANSACTION to undo changes.

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

4fill in blank

hard

Fill both blanks to create a savepoint and rollback to it.

MySQL

SAVEPOINT [1];
ROLLBACK TO [2];

Drag options to blanks, or click blank then click option'

Asp1

Btransaction1

Csave1

Drollback1

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for savepoint and rollback.

Using reserved words as savepoint names.

A savepoint named sp1 is created, and then the transaction rolls back to that savepoint, undoing changes after it.

5fill in blank

hard

Fill all three blanks to start a transaction, update a value, and commit the changes.

MySQL

START [1];
UPDATE accounts SET balance = balance [2] 100 WHERE id = 1;
[3];

Drag options to blanks, or click blank then click option'

ATRANSACTION

B+

CCOMMIT

DROLLBACK

Attempts:

3 left

💡 Hint

Common Mistakes

Using ROLLBACK instead of COMMIT to save changes.

Using '-' instead of '+' to add money.

This code starts a transaction, adds 100 to the balance of account with id 1, and then commits the changes to save them.

Practice

1. Why do transactions help maintain data integrity in MySQL?

easy

A. They speed up query execution by caching results.

B. They ensure all steps succeed or none do, avoiding partial updates.

C. They automatically backup the database after each query.

D. They allow multiple users to edit the same data simultaneously without locks.

Solution

1. Step 1: Understand the purpose of transactions

   Transactions group multiple database operations into a single unit that either fully succeeds or fully fails.

2. Step 2: Explain how this prevents partial updates

   If one step fails, the transaction rolls back all changes, preventing inconsistent or partial data states.

3. Final Answer:

   They ensure all steps succeed or none do, avoiding partial updates. -> Option B

4. Quick Check:

   All-or-nothing = data integrity [OK]

Hint: Remember: transactions are all-or-nothing actions [OK]

Common Mistakes:

• Thinking transactions speed up queries
• Believing transactions auto-backup data
• Assuming transactions allow unsafe simultaneous edits

2. Which of the following is the correct syntax to start a transaction in MySQL?

easy

A. START TRANSACTION;

B. OPEN TRANSACTION;

C. BEGIN TRANSACTION;

D. INIT TRANSACTION;

Solution

1. Step 1: Recall MySQL transaction start syntax

   MySQL uses START TRANSACTION; to begin a transaction explicitly.

2. Step 2: Compare options with correct syntax

   BEGIN TRANSACTION;, OPEN TRANSACTION;, and INIT TRANSACTION; are not recognized by MySQL for starting transactions.

3. Final Answer:

   START TRANSACTION; -> Option A

4. Quick Check:

   Start transaction command = START TRANSACTION [OK]

Hint: Use START TRANSACTION to begin safely [OK]

Common Mistakes:

• Using BEGIN TRANSACTION which is not valid in MySQL
• Confusing transaction commands with other SQL commands
• Assuming INIT TRANSACTION is correct syntax

3. Consider this sequence of SQL commands in MySQL:

START TRANSACTION;
UPDATE accounts SET balance = balance - 50 WHERE id = 1;
ROLLBACK;

What will be the balance of account with id=1 after these commands?

medium

A. NULL

B. 50

C. 0

D. 100

Solution

1. Step 1: Analyze the transaction commands

   The transaction starts, then subtracts 50 from the balance of account with id=1.

2. Step 2: Understand the effect of ROLLBACK

   ROLLBACK cancels all changes made in the transaction, so the update is undone.

3. Final Answer:

   100 -> Option D

4. Quick Check:

   Rollback cancels changes = balance stays 100 [OK]

Hint: ROLLBACK undoes all changes in the transaction [OK]

Common Mistakes:

• Assuming update persists after rollback
• Thinking balance becomes 50 after update
• Confusing ROLLBACK with COMMIT

4. You wrote this MySQL transaction:

START TRANSACTION;
UPDATE inventory SET quantity = quantity - 5 WHERE product_id = 10;
COMMIT;

But the quantity became negative. What is the likely problem?

medium

A. There was no check to prevent quantity from going below zero.

B. The transaction was not started properly.

C. COMMIT was used instead of ROLLBACK.

D. MySQL does not support transactions on UPDATE statements.

Solution

1. Step 1: Review the transaction logic

   The transaction updates quantity by subtracting 5 but does not check if quantity is already less than 5.

2. Step 2: Identify missing validation

   Without a condition to prevent negative values, quantity can become negative despite the transaction.

3. Final Answer:

   There was no check to prevent quantity from going below zero. -> Option A

4. Quick Check:

   Validation missing = negative quantity allowed [OK]

Hint: Always validate data before committing changes [OK]

Common Mistakes:

• Thinking COMMIT causes errors
• Believing MySQL disallows transactions on UPDATE
• Assuming transaction start failed without evidence

5. You want to transfer $200 from account A (id=1) to account B (id=2) safely using transactions. Which sequence ensures data integrity?

hard

A. START TRANSACTION; UPDATE accounts SET balance = balance - 200 WHERE id = 1; ROLLBACK; UPDATE accounts SET balance = balance + 200 WHERE id = 2; COMMIT;

B. UPDATE accounts SET balance = balance - 200 WHERE id = 1; UPDATE accounts SET balance = balance + 200 WHERE id = 2;

C. START TRANSACTION; UPDATE accounts SET balance = balance - 200 WHERE id = 1; UPDATE accounts SET balance = balance + 200 WHERE id = 2; COMMIT;

D. START TRANSACTION; UPDATE accounts SET balance = balance - 200 WHERE id = 1; COMMIT; UPDATE accounts SET balance = balance + 200 WHERE id = 2;

Solution

1. Step 1: Understand safe transfer with transactions

   Both debit and credit updates must be in one transaction to avoid partial transfer.

2. Step 2: Analyze each option

   START TRANSACTION; UPDATE accounts SET balance = balance - 200 WHERE id = 1; UPDATE accounts SET balance = balance + 200 WHERE id = 2; COMMIT; wraps both updates in a transaction and commits together, ensuring atomicity. UPDATE accounts SET balance = balance - 200 WHERE id = 1; UPDATE accounts SET balance = balance + 200 WHERE id = 2; lacks transaction, risking partial update. START TRANSACTION; UPDATE accounts SET balance = balance - 200 WHERE id = 1; COMMIT; UPDATE accounts SET balance = balance + 200 WHERE id = 2; commits debit separately, then credits outside transaction, risking partial update. START TRANSACTION; UPDATE accounts SET balance = balance - 200 WHERE id = 1; ROLLBACK; UPDATE accounts SET balance = balance + 200 WHERE id = 2; COMMIT; rolls back debit before credit, causing inconsistency.

3. Final Answer:

   START TRANSACTION; UPDATE accounts SET balance = balance - 200 WHERE id = 1; UPDATE accounts SET balance = balance + 200 WHERE id = 2; COMMIT; -> Option C

4. Quick Check:

   Atomic debit and credit in one transaction = START TRANSACTION; UPDATE accounts SET balance = balance - 200 WHERE id = 1; UPDATE accounts SET balance = balance + 200 WHERE id = 2; COMMIT; [OK]

Hint: Group related updates inside one transaction [OK]

Common Mistakes:

• Not using transactions for multi-step updates
• Rolling back part of the transaction incorrectly
• Updating accounts in wrong order without transaction