Transaction isolation levels in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of transaction isolation levels in databases?

Transaction isolation levels control how and when the changes made by one transaction become visible to other transactions, helping to prevent problems like dirty reads, non-repeatable reads, and phantom reads.

beginner

Name the four standard transaction isolation levels defined by SQL.

The four standard isolation levels are:
1. Read Uncommitted
2. Read Committed
3. Repeatable Read
4. Serializable

intermediate

What problem does the Read Uncommitted isolation level allow that others prevent?

Read Uncommitted allows dirty reads, meaning a transaction can see uncommitted changes made by other transactions, which might later be rolled back.

advanced

Explain the difference between Repeatable Read and Serializable isolation levels.

Repeatable Read prevents dirty and non-repeatable reads by ensuring data read once cannot change during the transaction, but phantom reads can still occur.
Serializable is the strictest level, preventing dirty reads, non-repeatable reads, and phantom reads by making transactions appear as if they run one after another.

intermediate

What is a phantom read in the context of transaction isolation?

A phantom read happens when a transaction reads a set of rows matching a condition, but a subsequent read in the same transaction finds new rows that were inserted by another committed transaction.

Which isolation level allows a transaction to read data that another transaction has modified but not yet committed?

A Read Uncommitted

B Read Committed

C Repeatable Read

D Serializable

Read Uncommitted allows dirty reads, meaning it can read uncommitted changes from other transactions.

Which isolation level guarantees that if you read the same row twice in a transaction, you get the same data both times?

A Read Uncommitted

B Read Committed

C Repeatable Read

D Serializable

Repeatable Read prevents non-repeatable reads by ensuring data read once cannot change during the transaction.

What problem does the Serializable isolation level prevent that Repeatable Read does not?

A Dirty reads

B Non-repeatable reads

C Lost updates

D Phantom reads

Serializable prevents phantom reads by making transactions appear to run one after another.

Which isolation level is the least strict and can lead to the most concurrency but also the most anomalies?

A Repeatable Read

B Read Uncommitted

C Read Committed

D Serializable

Read Uncommitted is the least strict and allows dirty reads, increasing concurrency but risking anomalies.

In which isolation level are changes made by other transactions only visible after they commit?

A Read Committed

B Repeatable Read

C Read Uncommitted

D Serializable

Read Committed only shows data from transactions that have committed, preventing dirty reads.

Describe the four standard transaction isolation levels and the types of read anomalies they prevent.

Think about how each level controls visibility of changes and what problems can happen.

Explain what phantom reads are and which isolation level prevents them.

Consider how new rows inserted by other transactions affect repeated queries.

Practice

1. Which transaction isolation level allows a transaction to see uncommitted changes made by other transactions?

easy

A. READ UNCOMMITTED

B. READ COMMITTED

C. REPEATABLE READ

D. SERIALIZABLE

Solution

1. Step 1: Understand isolation levels and identify the matching one

   READ UNCOMMITTED is the lowest isolation level allowing dirty reads (seeing uncommitted changes from other transactions).

2. Final Answer:

   READ UNCOMMITTED -> Option A

3. Quick Check:

   Dirty reads = READ UNCOMMITTED [OK]

Hint: Lowest level allows dirty reads: READ UNCOMMITTED [OK]

Common Mistakes:

• Confusing READ COMMITTED with allowing dirty reads
• Thinking SERIALIZABLE allows uncommitted reads
• Mixing REPEATABLE READ with dirty read behavior

2. Which of the following is the correct SQL syntax to set the transaction isolation level to SERIALIZABLE?

easy

A. SET ISOLATION LEVEL TRANSACTION SERIALIZABLE;

B. SET TRANSACTION LEVEL ISOLATION SERIALIZABLE;

C. SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;

D. SET SERIALIZABLE ISOLATION LEVEL;

Solution

1. Step 1: Recall and match the correct SQL syntax

   The standard syntax is SET TRANSACTION ISOLATION LEVEL [level]; Only SET TRANSACTION ISOLATION LEVEL SERIALIZABLE; matches exactly.

2. Final Answer:

   SET TRANSACTION ISOLATION LEVEL SERIALIZABLE; -> Option C

3. Quick Check:

   Correct syntax = SET TRANSACTION ISOLATION LEVEL SERIALIZABLE; [OK]

Hint: Remember: SET TRANSACTION ISOLATION LEVEL [level]; [OK]

Common Mistakes:

• Swapping keywords order incorrectly
• Omitting 'TRANSACTION' keyword
• Using non-standard syntax

3. Consider two transactions T1 and T2 running concurrently. T1 reads a row, then T2 updates and commits that row. If the isolation level is REPEATABLE READ, what will T1 see when it reads the same row again?

medium

A. The updated value committed by T2

B. The original value before T2's update

C. An error due to concurrent update

D. A partially updated value

Solution

1. Step 1: Understand REPEATABLE READ and apply to scenario

   REPEATABLE READ ensures a transaction sees the same data on repeated reads of the same row. T1's second read sees the original value despite T2's committed update.

2. Final Answer:

   The original value before T2's update -> Option B

3. Quick Check:

   REPEATABLE READ prevents non-repeatable reads = The original value before T2's update [OK]

Hint: REPEATABLE READ keeps data stable during transaction [OK]

Common Mistakes:

• Assuming T1 sees T2's committed update immediately
• Confusing REPEATABLE READ with READ COMMITTED
• Thinking it causes errors on concurrent updates

4. You wrote this SQL command to set the isolation level but get an error:
SET TRANSACTION LEVEL ISOLATION READ COMMITTED;
What is the error in this statement?

medium

A. The statement is correct; error is elsewhere.

B. 'TRANSACTION' keyword is missing.

C. 'READ COMMITTED' is not a valid isolation level.

D. The order of 'LEVEL' and 'ISOLATION' is incorrect.

Solution

1. Step 1: Check correct syntax and identify the error

   Correct syntax is SET TRANSACTION ISOLATION LEVEL [level]; The given statement swaps 'LEVEL' and 'ISOLATION', causing a syntax error.

2. Final Answer:

   The order of 'LEVEL' and 'ISOLATION' is incorrect. -> Option D

3. Quick Check:

   Syntax order matters = The order of 'LEVEL' and 'ISOLATION' is incorrect. [OK]

Hint: Remember correct keyword order: ISOLATION LEVEL [OK]

Common Mistakes:

• Swapping 'LEVEL' and 'ISOLATION' keywords
• Assuming 'READ COMMITTED' is invalid
• Missing semicolon causing error

5. A bank application uses SERIALIZABLE isolation level to prevent anomalies. However, users report frequent transaction failures due to serialization errors. What is the best approach to reduce these errors while maintaining data accuracy?

hard

A. Keep SERIALIZABLE and retry failed transactions automatically

B. Disable transactions to improve performance

C. Switch to READ UNCOMMITTED to avoid errors

D. Lower isolation level to READ COMMITTED and add explicit locking where needed

Solution

1. Step 1: Understand SERIALIZABLE issues and evaluate best practice

   SERIALIZABLE prevents anomalies but causes serialization errors on conflicts. Lowering isolation risks anomalies; best is to keep SERIALIZABLE and retry failed transactions automatically.

2. Final Answer:

   Keep SERIALIZABLE and retry failed transactions automatically -> Option A

3. Quick Check:

   Retry on serialization errors = Keep SERIALIZABLE and retry failed transactions automatically [OK]

Hint: Retry failed SERIALIZABLE transactions to keep accuracy [OK]

Common Mistakes:

• Lowering isolation level without safeguards
• Ignoring serialization errors
• Disabling transactions for performance