Can you list the different levels of transaction isolation in database systems?

In database systems, four main levels of transaction isolation are usually distinguished, as defined by the SQL standard:

1. Read Uncommitted — the lowest level of isolation. A transaction can see changes made by other transactions even if they are not yet committed (dirty reads).

2. Read Committed — a transaction only sees data that has been committed by other transactions. This prevents dirty reads but allows non-repeatable reads.

3. Repeatable Read — guarantees that if a transaction has read data, subsequent reads of the same data will yield the same result, preventing non-repeatable reads. However, phantom reads are still possible.

4. Serializable — the strictest level, where transactions are executed as if they were sequential, one after another. This prevents all types of read anomalies, including phantom reads.

Each isolation level balances between performance and data integrity.