Deadlock concept and prevention in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start a transaction in SQL.

SQL

BEGIN [1];

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

AROLLBACK

BTRANSACTION

CCOMMIT

DSAVEPOINT

Attempts:

3 left

💡 Hint

Common Mistakes

Using COMMIT or ROLLBACK to start a transaction instead of ending it.

Using SAVEPOINT to start a transaction.

To start a transaction in SQL, you use BEGIN TRANSACTION.

2fill in blank

medium

Complete the code to lock a table for writing in SQL.

SQL

LOCK TABLE [1] IN EXCLUSIVE MODE;

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

ACOMMIT

BSELECT

Cusers

DBEGIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using SQL keywords like SELECT or COMMIT instead of a table name.

Leaving the table name blank.

The LOCK TABLE statement locks a specific table, here the table name is needed.

3fill in blank

hard

Fix the error in the SQL code to prevent deadlock by setting the transaction isolation level.

SQL

SET TRANSACTION ISOLATION LEVEL [1];

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ASERIALIZABLE

BREAD UNCOMMITTED

CREAD COMMITTED

DDIRTY READ

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing lower isolation levels like READ UNCOMMITTED which allow dirty reads.

Using invalid isolation levels like DIRTY READ.

Using SERIALIZABLE isolation level helps prevent deadlocks by ensuring transactions are fully isolated.

4fill in blank

hard

Fill both blanks to create a query that detects blocking sessions causing deadlocks.

SQL

SELECT blocking_session_id, [1] FROM sys.dm_exec_requests WHERE [2] > 0;

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Asession_id

Btransaction_id

Cblocking_session_id

Dlock_owner_address

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same column for both blanks.

Using unrelated columns that do not indicate blocking.

The query selects the blocking session and the session being blocked to identify deadlocks.

5fill in blank

hard

Fill all three blanks to write a query that kills a blocking session to resolve deadlock.

SQL

DECLARE @session_id INT = ([1] TOP 1 blocking_session_id FROM sys.dm_exec_requests WHERE blocking_session_id > 0);
IF EXISTS (SELECT 1 FROM sys.dm_exec_sessions WHERE session_id = @session_id)
BEGIN
  [2] [3] @session_id;
END

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

AKILL

BSELECT

CEXEC

DDROP

Attempts:

3 left

💡 Hint

Common Mistakes

Using DROP instead of KILL to end a session.

Not checking if the session exists before killing.

The query assigns a session id using a subquery to find a blocking session, checks if it exists, then executes the KILL command to end the blocking session.

Practice

1. What is a deadlock in a database system?

easy

A. A backup process that locks the entire database

B. A syntax error in SQL queries causing failure

C. A method to speed up query execution

D. A situation where two or more transactions wait indefinitely for each other to release locks

Solution

1. Step 1: Understand transaction locking

   Transactions lock resources to maintain data integrity during operations.

2. Step 2: Identify deadlock condition

   Deadlock occurs when transactions wait on each other's locked resources indefinitely, causing a standstill.

3. Final Answer:

   A situation where two or more transactions wait indefinitely for each other to release locks -> Option D

4. Quick Check:

   Deadlock = Transactions waiting forever [OK]

Hint: Deadlock means waiting forever for each other's locked resource [OK]

Common Mistakes:

• Confusing deadlock with syntax errors
• Thinking deadlock speeds up queries
• Assuming deadlock is a backup process

2. Which SQL statement is best to prevent deadlocks by controlling transaction order?

easy

A. Run transactions without COMMIT statements

B. Use SELECT * without WHERE clause

C. Always access tables in the same order in all transactions

D. Use DROP TABLE before INSERT

Solution

1. Step 1: Understand deadlock prevention strategy

   Accessing resources in a consistent order prevents circular waiting among transactions.

2. Step 2: Evaluate options

   Only accessing tables in the same order avoids deadlocks; other options cause errors or do not help.

3. Final Answer:

   Always access tables in the same order in all transactions -> Option C

4. Quick Check:

   Consistent access order = Deadlock prevention [OK]

Hint: Access tables in same order to avoid deadlocks [OK]

Common Mistakes:

• Ignoring transaction commit timing
• Using SELECT * without filters causes performance issues, not deadlocks
• Dropping tables is unrelated to deadlock prevention

3. Consider two transactions:
Transaction 1: LOCK table A; LOCK table B;
Transaction 2: LOCK table B; LOCK table A;
What is the likely outcome?

medium

A. Deadlock occurs because each waits for the other's locked table

B. Transaction 2 commits immediately

C. Transaction 1 fails due to syntax error

D. Both transactions complete successfully without waiting

Solution

1. Step 1: Analyze lock order in transactions

   Transaction 1 locks table A then B; Transaction 2 locks table B then A, creating circular wait.

2. Step 2: Identify deadlock condition

   Each transaction waits for the other's locked resource, causing deadlock.

3. Final Answer:

   Deadlock occurs because each waits for the other's locked table -> Option A

4. Quick Check:

   Opposite lock order = Deadlock [OK]

Hint: Opposite lock order in transactions causes deadlock [OK]

Common Mistakes:

• Assuming transactions run without waiting
• Confusing deadlock with syntax errors
• Thinking one transaction commits immediately

4. You have a deadlock error in your SQL transactions. Which fix will most likely resolve it?

medium

A. Remove all COMMIT statements

B. Change transactions to lock tables in the same order

C. Use SELECT without WHERE clause to avoid locks

D. Increase transaction isolation level to SERIALIZABLE

Solution

1. Step 1: Identify cause of deadlock

   Deadlocks often happen due to inconsistent lock order in transactions.

2. Step 2: Apply fix by ordering locks

   Changing transactions to lock tables in the same order prevents circular waiting and deadlocks.

3. Final Answer:

   Change transactions to lock tables in the same order -> Option B

4. Quick Check:

   Consistent lock order fixes deadlocks [OK]

Hint: Fix deadlocks by ordering locks consistently [OK]

Common Mistakes:

• Removing COMMIT causes longer locks, worsening deadlocks
• SELECT without WHERE does not prevent locks
• Higher isolation can increase deadlocks

5. You want to prevent deadlocks in a system where multiple transactions update two tables, Orders and Customers. Which approach is best?

hard

A. Ensure all transactions update Customers first, then Orders, and keep transactions short

B. Allow transactions to update tables in any order but increase isolation level to SERIALIZABLE

C. Use long transactions with many updates to reduce lock frequency

D. Disable locking to speed up updates

Solution

1. Step 1: Understand deadlock prevention principles

   Accessing resources in the same order and keeping transactions short reduces deadlocks.

2. Step 2: Evaluate options for best practice

   Ensure all transactions update Customers first, then Orders, and keep transactions short enforces consistent order and short transactions, preventing deadlocks effectively.

3. Step 3: Analyze other options

   Allow transactions to update tables in any order but increase isolation level to SERIALIZABLE may increase deadlocks; Use long transactions with many updates to reduce lock frequency lengthens locks; Disable locking to speed up updates is unsafe and invalid.

4. Final Answer:

   Ensure all transactions update Customers first, then Orders, and keep transactions short -> Option A

5. Quick Check:

   Consistent order + short transactions = Deadlock prevention [OK]

Hint: Use same update order and short transactions to prevent deadlocks [OK]

Common Mistakes:

• Increasing isolation level can cause more deadlocks
• Long transactions hold locks longer, increasing deadlock risk
• Disabling locking breaks data integrity