Transaction isolation levels in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set the transaction isolation level to READ COMMITTED.

SQL

SET TRANSACTION ISOLATION LEVEL [1];

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

AREAD UNCOMMITTED

BSERIALIZABLE

CREPEATABLE READ

DREAD COMMITTED

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing READ UNCOMMITTED causes dirty reads.

Choosing SERIALIZABLE is stricter than needed here.

The READ COMMITTED level allows a transaction to see only committed data, preventing dirty reads.

2fill in blank

medium

Complete the code to start a transaction with the SERIALIZABLE isolation level.

SQL

START TRANSACTION ISOLATION LEVEL [1];

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

ASERIALIZABLE

BREAD COMMITTED

CREPEATABLE READ

DREAD UNCOMMITTED

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing REPEATABLE READ does not prevent phantom reads.

Choosing READ COMMITTED allows some concurrency anomalies.

SERIALIZABLE is the strictest isolation level, preventing all concurrency anomalies.

3fill in blank

hard

Fix the error in the code to set the isolation level to REPEATABLE READ.

SQL

SET TRANSACTION ISOLATION [1] REPEATABLE READ;

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

ATO

BMODE

CLEVEL

DWITH

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'MODE' instead of 'LEVEL' causes syntax errors.

Omitting the keyword after ISOLATION is invalid.

The correct syntax is SET TRANSACTION ISOLATION LEVEL followed by the level name.

4fill in blank

hard

Fill the three blanks to complete the command that sets the isolation level to READ UNCOMMITTED for the current session.

SQL

SET SESSION TRANSACTION ISOLATION [1] [2] [3];

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ALEVEL

BREAD

CUNCOMMITTED

DMODE

Attempts:

3 left

💡 Hint

Common Mistakes

Using MODE instead of LEVEL causes errors.

Missing the isolation level name after LEVEL.

The correct syntax is SET SESSION TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;. This is the least strict isolation level, allowing dirty reads.

5fill in blank

hard

Fill all three blanks to write a query that shows the current transaction isolation level.

SQL

SELECT @@transaction_isolation AS [1], @@[2] AS [3];

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

Acurrent_level

Btx_isolation

Csession_level

Disolation_level

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect variable names causes errors.

Not aliasing columns makes output less clear.

This query selects the current transaction isolation level using system variables. '@@transaction_isolation' is standard, '@@tx_isolation' is a synonym in some systems, and aliasing helps readability.

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