Isolation levels in MySQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

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

MySQL

SET TRANSACTION ISOLATION LEVEL [1];

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

AREAD COMMITTED

BREPEATABLE READ

CREAD UNCOMMITTED

DSERIALIZABLE

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing READ UNCOMMITTED which allows dirty reads.

Using SERIALIZABLE which is stricter than needed.

The READ COMMITTED isolation level ensures that any data read is committed at the moment it is read. This prevents dirty reads.

2fill in blank

medium

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

MySQL

START TRANSACTION ISOLATION LEVEL [1];

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

AREAD UNCOMMITTED

BREAD COMMITTED

CSERIALIZABLE

DREPEATABLE READ

Attempts:

3 left

💡 Hint

Common Mistakes

Using REPEATABLE READ which is less strict.

Forgetting to specify the isolation level in the transaction start.

SERIALIZABLE is the strictest isolation level, preventing phantom reads and ensuring full transaction isolation.

3fill in blank

hard

Fix the error in the code to set the isolation level globally.

MySQL

SET GLOBAL TRANSACTION ISOLATION LEVEL [1];

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

AREPEATABLE READ

BREAD UNCOMMITTED

CREAD COMMITTED

DSERIALIZABLE

Attempts:

3 left

💡 Hint

Common Mistakes

Using an unsupported isolation level for global setting.

Syntax errors in the SET GLOBAL command.

The default global isolation level in MySQL is REPEATABLE READ. This command sets it correctly.

4fill in blank

hard

Fill both blanks to complete the query that shows the current transaction isolation level.

MySQL

SELECT @@transaction_isolation AS [1], @@global.transaction_isolation AS [2];

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

Acurrent_level

Bglobal_level

Cisolation_level

Dlevel

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same alias for both columns.

Using non-descriptive alias names.

The query returns the current session isolation level as 'isolation_level' and the global isolation level as 'global_level'.

5fill in blank

hard

Fill the blanks to complete the code that sets the isolation level for the next transaction and then starts it.

MySQL

SET SESSION TRANSACTION ISOLATION LEVEL [1]; START TRANSACTION [2];

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

AREAD COMMITTED

BISOLATION LEVEL SERIALIZABLE

CREAD UNCOMMITTED

DISOLATION LEVEL READ COMMITTED

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing isolation level keywords incorrectly.

Not specifying isolation level in START TRANSACTION.

The first statement sets the session isolation level to READ COMMITTED. The second starts a transaction with SERIALIZABLE isolation level specified explicitly.

Practice

1. Which isolation level allows a transaction to read data that has been modified by other transactions but not yet committed?

easy

A. REPEATABLE READ

B. READ COMMITTED

C. READ UNCOMMITTED

D. SERIALIZABLE

Solution

1. Step 1: Understand isolation levels and dirty reads

   READ UNCOMMITTED allows reading uncommitted changes, causing dirty reads.

2. Step 2: Compare with other levels

   Other levels prevent dirty reads by requiring commits before reading.

3. Final Answer:

   READ UNCOMMITTED -> Option C

4. Quick Check:

   Dirty reads allowed = READ UNCOMMITTED [OK]

Hint: Dirty reads happen only in READ UNCOMMITTED [OK]

Common Mistakes:

• Confusing READ COMMITTED with allowing dirty reads
• Thinking SERIALIZABLE allows dirty reads
• Assuming REPEATABLE READ reads uncommitted data

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

easy

A. SET ISOLATION SERIALIZABLE;

B. SET ISOLATION LEVEL = SERIALIZABLE;

C. SET TRANSACTION LEVEL SERIALIZABLE;

D. SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;

Solution

1. Step 1: Recall correct syntax for setting isolation level

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

2. Step 2: Check each option

   Only SET TRANSACTION ISOLATION LEVEL SERIALIZABLE; matches the correct syntax exactly.

3. Final Answer:

   SET TRANSACTION ISOLATION LEVEL SERIALIZABLE; -> Option D

4. Quick Check:

   Correct syntax starts with SET TRANSACTION ISOLATION LEVEL [OK]

Hint: Use full phrase: SET TRANSACTION ISOLATION LEVEL [OK]

Common Mistakes:

• Omitting TRANSACTION keyword
• Using = sign incorrectly
• Shortening syntax incorrectly

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 from T2

B. The original value before T2's update

C. An error due to conflict

D. A partially updated value

Solution

1. Step 1: Understand REPEATABLE READ behavior

   REPEATABLE READ ensures that repeated reads in the same transaction see the same data snapshot.

2. Step 2: Apply to scenario

   T1 will see the original value it read first, ignoring T2's committed update during T1's transaction.

3. Final Answer:

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

4. Quick Check:

   REPEATABLE READ = consistent snapshot reads [OK]

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

Common Mistakes:

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

4. You set the isolation level to READ COMMITTED but notice phantom reads occurring. What is the likely cause?

medium

A. READ COMMITTED does not prevent phantom reads

B. The isolation level was not set correctly

C. Phantom reads only occur in READ UNCOMMITTED

D. The database engine does not support transactions

Solution

1. Step 1: Recall what phantom reads are

   Phantom reads happen when new rows appear in repeated queries within a transaction.

2. Step 2: Understand READ COMMITTED limits

   READ COMMITTED prevents dirty reads but does not prevent phantom reads.

3. Final Answer:

   READ COMMITTED does not prevent phantom reads -> Option A

4. Quick Check:

   Phantom reads allowed in READ COMMITTED [OK]

Hint: Phantom reads happen above READ COMMITTED level [OK]

Common Mistakes:

• Assuming READ COMMITTED blocks phantom reads
• Blaming incorrect isolation level setting
• Ignoring database engine capabilities

5. You want to ensure no dirty reads, no non-repeatable reads, and no phantom reads in a high-concurrency environment. Which isolation level should you choose and why?

hard

A. SERIALIZABLE, because it prevents all concurrency anomalies

B. READ UNCOMMITTED, because it allows fastest access

C. READ COMMITTED, because it balances speed and accuracy

D. REPEATABLE READ, because it prevents dirty and phantom reads

Solution

1. Step 1: Identify isolation level requirements

   No dirty reads, no non-repeatable reads, and no phantom reads means highest isolation.

2. Step 2: Match requirements to isolation levels

   SERIALIZABLE is the strictest level preventing all these anomalies by serializing transactions.

3. Step 3: Compare with other levels

   READ COMMITTED and REPEATABLE READ allow some anomalies; READ UNCOMMITTED allows dirty reads.

4. Final Answer:

   SERIALIZABLE, because it prevents all concurrency anomalies -> Option A

5. Quick Check:

   SERIALIZABLE = strictest isolation, no anomalies [OK]

Hint: Choose SERIALIZABLE for full isolation and no anomalies [OK]

Common Mistakes:

• Choosing REPEATABLE READ and ignoring phantom reads
• Picking READ COMMITTED for full isolation
• Confusing speed with isolation guarantees