SQL Transaction Isolation Levels

Transaction isolation levels define the degree to which the operations in one transaction are visible to other concurrent transactions and the types of reads and writes that are allowed.

Comparison Table

Explanation of Isolation Levels

1. READ UNCOMMITTED

This is the lowest isolation level. Transactions can read data that has been modified by other transactions but not yet committed.

• Allows dirty reads, non-repeatable reads, and phantom reads
• Provides the highest level of concurrency
• Least consistent data state

2. READ COMMITTED

This level guarantees that any data read was committed at the moment it was read. It prevents dirty reads.

• Prevents dirty reads
• Allows non-repeatable reads and phantom reads
• Each consistent read sets and reads its own fresh snapshot

3. REPEATABLE READ

This level guarantees that any data read cannot change if the transaction reads it again. It prevents non-repeatable reads.

• Prevents dirty reads and non-repeatable reads
• Allows phantom reads
• All consistent reads within the same transaction read the snapshot established by the first read

4. SERIALIZABLE

This is the highest isolation level. Transactions are executed as if they were run sequentially.

• Prevents dirty reads, non-repeatable reads, and phantom reads
• Provides the highest level of isolation
• May significantly impact performance due to increased locking

Phenomena Explained

• Dirty Read: A transaction reads data that has not yet been committed.
• Non-repeatable Read: A transaction reads the same row twice and gets different data each time.
• Phantom Read: A transaction re-executes a query returning a set of rows that satisfy a search condition and finds that the set of rows satisfying the condition has changed due to another recently-committed transaction.