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

Transaction isolation levels determine how much one transaction can see the changes made by other concurrent transactions. The main standardized SQL isolation levels are:

1. Read Uncommitted — the lowest level. A transaction can see uncommitted changes made by others (dirty read). This can lead to unpredictable results.

2. Read Committed — a transaction only sees data that has been committed by others. Dirty reads are prevented, but non-repeatable reads can occur, where data may change upon re-reading.

3. Repeatable Read — guarantees that data read during a transaction will not change until the transaction completes. It prevents non-repeatable reads but allows phantom reads, where new rows appear upon re-query.

4. Serializable — the strictest level. Transactions are executed as if they were sequential, one after another. It prevents all anomalies, including phantom reads, but reduces concurrency and performance.

Choosing an isolation level is a trade-off between data integrity and system performance.