Transaction isolation levels in SQL

Introduction

Transaction isolation levels control how much one transaction sees changes made by others. They help keep data correct when many people use the database at the same time.

When multiple users update the same data at once and you want to avoid mistakes.

When you want to balance speed and accuracy in your database operations.

When you need to prevent errors like reading data that is being changed by others.

When you want to control how strict the database is about seeing uncommitted changes.

When designing a system that handles money or important records where accuracy matters.

Syntax

SQL

SET TRANSACTION ISOLATION LEVEL level_name;

Replace level_name with one of the standard levels: READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, or SERIALIZABLE.

This command sets the isolation level for the current transaction or session depending on the database.

Examples

This level allows reading only committed data, avoiding dirty reads.

SQL

SET TRANSACTION ISOLATION LEVEL READ COMMITTED;

This is the strictest level, preventing all concurrency problems but can slow down performance.

SQL

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;

This level allows reading uncommitted changes, which can be faster but may show incorrect data.

SQL

SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;

Sample Program

This example sets the isolation level to REPEATABLE READ, which ensures that if you read the same data again in this transaction, it won't change.

SQL

-- Set isolation level to REPEATABLE READ
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;

-- Start a transaction
BEGIN TRANSACTION;

-- Example query
SELECT * FROM Orders WHERE CustomerID = 1;

-- Commit transaction
COMMIT;

Important Notes

Higher isolation levels reduce errors but can slow down the system because transactions wait for each other.

Not all databases support all isolation levels exactly the same way.

Choosing the right isolation level depends on your application's needs for accuracy and speed.

Summary

Transaction isolation levels control how transactions see each other's changes.

Common levels are READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, and SERIALIZABLE.

Higher levels give more accuracy but can reduce performance.

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