Read phenomena (dirty reads, phantom reads) in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What output shows a dirty read scenario?

Consider two transactions:

Transaction 1 updates a row but does not commit.
Transaction 2 reads the updated row before commit.

Which query result best illustrates the dirty read phenomenon?

SQL

BEGIN TRANSACTION;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
-- no commit yet

-- In parallel
SELECT balance FROM accounts WHERE id = 1;

A Transaction 2 reads the old balance before update.

B Transaction 2 reads a balance from a different account.

C Transaction 2 waits until Transaction 1 commits before reading.

D Transaction 2 reads the new balance even though Transaction 1 has not committed.

💡 Hint

Dirty reads happen when a transaction reads uncommitted changes from another transaction.

A dirty read occurs when a transaction reads data that has been modified by another transaction but not yet committed. Option D describes this situation.

❓ query_result

intermediate

Which output best illustrates a phantom read?

Transaction 1 runs a query to count rows matching a condition.
Transaction 2 inserts a new row matching that condition and commits.
Transaction 1 runs the same query again.

What is the expected result?

SQL

BEGIN TRANSACTION;
SELECT COUNT(*) FROM orders WHERE status = 'pending';
-- returns 5

-- In parallel
INSERT INTO orders (id, status) VALUES (101, 'pending');
COMMIT;

-- Back in Transaction 1
SELECT COUNT(*) FROM orders WHERE status = 'pending';

A First query returns 5, second query returns 6 rows.

B Both queries return 5 rows.

C Both queries return 6 rows.

D Second query returns fewer rows than the first.

💡 Hint

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

Phantom reads occur when a transaction re-executes a query and sees new rows inserted by another committed transaction. Here, the count changes from 5 to 6.

🧠 Conceptual

advanced

Which isolation level prevents dirty reads but allows phantom reads?

Choose the SQL transaction isolation level that stops dirty reads but still permits phantom reads.

A READ UNCOMMITTED

B SERIALIZABLE

C READ COMMITTED

D REPEATABLE READ

💡 Hint

Think about which isolation level blocks reading uncommitted data but does not lock range scans.

READ COMMITTED prevents dirty reads by only reading committed data but does not prevent phantom reads because new rows can appear in repeated queries.

📝 Syntax

advanced

Which SQL snippet correctly sets the isolation level to prevent phantom reads?

You want to set the transaction isolation level to SERIALIZABLE to avoid phantom reads.
Which SQL command is correct?

A SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;

B SET ISOLATION LEVEL TRANSACTION SERIALIZABLE;

C SET TRANSACTION LEVEL ISOLATION SERIALIZABLE;

D SET ISOLATION SERIALIZABLE TRANSACTION LEVEL;

💡 Hint

The correct syntax follows the pattern: SET TRANSACTION ISOLATION LEVEL ...

Option A uses the correct standard SQL syntax to set the isolation level to SERIALIZABLE.

🔧 Debug

expert

Why does this transaction still experience phantom reads despite using REPEATABLE READ?

Transaction 1:
BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;
SELECT * FROM products WHERE category = 'books';
-- returns 10 rows

Transaction 2:
INSERT INTO products (id, category) VALUES (101, 'books');
COMMIT;

Transaction 1:
SELECT * FROM products WHERE category = 'books';
-- returns 11 rows

Why does Transaction 1 see the new row?

A The isolation level was not set correctly before the transaction started.

B REPEATABLE READ does not prevent phantom reads in all databases.

C Transaction 1 did not lock the table explicitly.

D The new row was inserted with a different category.

💡 Hint

Some databases implement REPEATABLE READ differently regarding phantom reads.

In some database systems like MySQL InnoDB, REPEATABLE READ prevents non-repeatable reads but phantom reads can still occur because it does not lock range scans fully.

Practice

1. Which of the following best describes a dirty read in database transactions?

easy

A. Reading data that has been modified but not yet committed by another transaction.

B. Seeing new rows appear in a query result after re-running the same query.

C. Reading only committed data and never uncommitted changes.

D. Locking rows to prevent other transactions from reading them.

Solution

1. Step 1: Understand dirty read definition

   A dirty read happens when a transaction reads data that another transaction has changed but not committed yet.

2. Step 2: Compare options to definition

   Reading data that has been modified but not yet committed by another transaction. matches this exactly. Seeing new rows appear in a query result after re-running the same query. describes phantom reads, C describes a safe read, and D is about locking, not dirty reads.

3. Final Answer:

   Reading data that has been modified but not yet committed by another transaction. -> Option A

4. Quick Check:

   Dirty read = reading uncommitted data [OK]

Hint: Dirty read = reading uncommitted changes [OK]

Common Mistakes:

• Confusing dirty read with phantom read
• Thinking dirty read only reads committed data
• Mixing locking with read phenomena

2. Which SQL transaction isolation level allows dirty reads?

easy

A. SERIALIZABLE

B. REPEATABLE READ

C. READ COMMITTED

D. READ UNCOMMITTED

Solution

1. Step 1: Recall isolation levels and dirty reads

   Only the READ UNCOMMITTED isolation level allows dirty reads, meaning it can read uncommitted changes.

2. Step 2: Match options to isolation levels

   READ COMMITTED and higher levels prevent dirty reads. SERIALIZABLE is the strictest. So, READ UNCOMMITTED is correct.

3. Final Answer:

   READ UNCOMMITTED -> Option D

4. Quick Check:

   Dirty reads allowed only in READ UNCOMMITTED [OK]

Hint: Dirty reads only happen at READ UNCOMMITTED level [OK]

Common Mistakes:

• Choosing READ COMMITTED as allowing dirty reads
• Confusing phantom reads with dirty reads
• Not knowing isolation level hierarchy

3. Consider two transactions running concurrently on a table Orders. Transaction 1 reads all rows where status = 'pending'. Transaction 2 inserts a new row with status = 'pending' and commits. If Transaction 1 repeats the read, what phenomenon might it observe?

medium

A. Dirty read

B. Non-repeatable read

C. Phantom read

D. Lost update

Solution

1. Step 1: Analyze the scenario with inserted rows

   Transaction 1 reads rows with status 'pending'. Transaction 2 inserts a new 'pending' row and commits.

2. Step 2: Identify the phenomenon on repeated read

   When Transaction 1 reads again, it sees the new row that wasn't there before. This is a phantom read, where new rows appear between reads.

3. Final Answer:

   Phantom read -> Option C

4. Quick Check:

   New rows appearing = Phantom read [OK]

Hint: New rows between reads = phantom read [OK]

Common Mistakes:

• Confusing phantom read with dirty read
• Thinking non-repeatable read applies to new rows
• Mixing lost update with read phenomena

4. A developer wrote this SQL transaction code but observes inconsistent reads. Identify the error related to read phenomena:

BEGIN TRANSACTION;
SELECT * FROM Products WHERE price > 100;
-- Another transaction inserts a new product with price 150 and commits
SELECT * FROM Products WHERE price > 100;
COMMIT;

What is the likely cause?

medium

A. Using READ UNCOMMITTED isolation level causing dirty reads

B. Using READ COMMITTED isolation level causing phantom reads

C. Using SERIALIZABLE isolation level causing lost updates

D. Using REPEATABLE READ isolation level causing non-repeatable reads

Solution

1. Step 1: Understand the scenario with repeated SELECTs

   The transaction reads products with price > 100 twice. Between reads, another transaction inserts a new matching row and commits.

2. Step 2: Identify the read phenomenon

   Because the isolation level is likely READ COMMITTED, the second read sees the new row, causing a phantom read.

3. Final Answer:

   Using READ COMMITTED isolation level causing phantom reads -> Option B

4. Quick Check:

   Phantom reads occur at READ COMMITTED [OK]

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

Common Mistakes:

• Assuming dirty reads cause this behavior
• Confusing lost updates with read phenomena
• Thinking REPEATABLE READ allows phantom reads

5. You want to prevent both dirty reads and phantom reads in your application. Which transaction isolation level should you choose?

hard

A. SERIALIZABLE

B. READ COMMITTED

C. REPEATABLE READ

D. READ UNCOMMITTED

Solution

1. Step 1: Review isolation levels and phenomena prevented

   READ UNCOMMITTED allows dirty reads. READ COMMITTED prevents dirty reads but allows phantom reads. REPEATABLE READ prevents dirty and non-repeatable reads but may allow phantom reads.

2. Step 2: Identify level preventing both dirty and phantom reads

   SERIALIZABLE is the strictest level and prevents dirty reads, non-repeatable reads, and phantom reads by fully isolating transactions.

3. Final Answer:

   SERIALIZABLE -> Option A

4. Quick Check:

   SERIALIZABLE prevents dirty and phantom reads [OK]

Hint: Use SERIALIZABLE to prevent dirty and phantom reads [OK]

Common Mistakes:

• Choosing REPEATABLE READ thinking it blocks phantom reads
• Confusing READ COMMITTED with preventing all read anomalies
• Ignoring performance impact of SERIALIZABLE