Read phenomena (dirty reads, phantom reads) in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all rows from the orders table.

SQL

SELECT * FROM [1];

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

Acustomers

Bemployees

Cproducts

Dorders

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting from the wrong table like 'customers' or 'products'.

Forgetting to specify the table name.

The orders table contains the data we want to read, so we select from it.

2fill in blank

medium

Complete the code to start a transaction with the isolation level that prevents dirty reads.

SQL

SET 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 READ UNCOMMITTED which allows dirty reads.

Confusing REPEATABLE READ or SERIALIZABLE with preventing dirty reads.

The READ COMMITTED isolation level prevents dirty reads by only reading committed data.

3fill in blank

hard

Fix the error in the transaction code to prevent phantom reads.

SQL

BEGIN TRANSACTION;
SET TRANSACTION ISOLATION LEVEL [1];
-- perform queries
COMMIT;

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

AREAD UNCOMMITTED

BREPEATABLE READ

CREAD COMMITTED

DDIRTY READ

Attempts:

3 left

💡 Hint

Common Mistakes

Using READ COMMITTED which does not prevent phantom reads.

Using invalid isolation level names like DIRTY READ.

The REPEATABLE READ isolation level prevents phantom reads by ensuring consistent reads within a transaction.

4fill in blank

hard

Fill both blanks to write a query that counts orders with amount greater than 100 inside a repeatable read transaction.

SQL

BEGIN TRANSACTION;
SET TRANSACTION ISOLATION LEVEL [1];
SELECT COUNT(*) FROM orders WHERE amount [2] 100;
COMMIT;

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

AREPEATABLE READ

B>

C<

DREAD COMMITTED

Attempts:

3 left

💡 Hint

Common Mistakes

Using READ COMMITTED which allows phantom reads.

Using '<' instead of '>' for the amount condition.

We use REPEATABLE READ to prevent phantom reads and > to select orders with amount greater than 100.

5fill in blank

hard

Fill all three blanks to write a transaction that prevents dirty reads and phantom reads, and selects orders with status 'pending'.

SQL

BEGIN TRANSACTION;
SET TRANSACTION ISOLATION LEVEL [1];
SELECT * FROM orders WHERE status = '[2]' AND amount [3] 0;
COMMIT;

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

ASERIALIZABLE

Bpending

C>

DREAD COMMITTED

Attempts:

3 left

💡 Hint

Common Mistakes

Using READ COMMITTED which does not prevent phantom reads.

Using wrong status value or comparison operator.

SERIALIZABLE is the strictest isolation level preventing dirty and phantom reads. We filter orders with status 'pending' and amount greater than 0.

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