What levels of transaction isolation exist and how do they differ?

Transaction isolation levels determine how and when the changes made by one transaction become visible to other transactions. The SQL standard defines four main levels of isolation:

1. Read Uncommitted

   • A transaction can see changes made by other transactions even if they have not been committed.
   • Dirty reads, non-repeatable reads, and phantom reads are possible.

2. Read Committed

   • A transaction only sees data that has been committed by other transactions.
   • Dirty reads are prevented, but non-repeatable and phantom reads can occur.

3. Repeatable Read

   • Ensures that if a transaction reads data, it will see the same data if it reads again.
   • Prevents dirty and non-repeatable reads, but phantom reads can occur.

4. Serializable

   • The strictest level of isolation.
   • Transactions are executed as if they were executed sequentially, one after another.
   • Prevents dirty, non-repeatable, and phantom reads.

Each isolation level is a compromise between performance and data integrity. For example, Read Uncommitted is the fastest but least safe, while Serializable is the safest but may reduce concurrency and performance.