List the transaction isolation levels and briefly explain their purposes.

Transaction isolation levels determine how much a transaction can see the changes made by other transactions and help control concurrency issues such as dirty reads, non-repeatable reads, and phantom reads.

Main isolation levels:

1. READ UNCOMMITTED — the lowest level. A transaction can see uncommitted changes made by other transactions (dirty read).
2. READ COMMITTED — a transaction only sees committed changes. Dirty reads are prevented, but non-repeatable reads can occur.
3. REPEATABLE READ — guarantees that repeated reads of the same data within a transaction will be identical. It prevents non-repeatable reads, but phantom reads are possible.
4. SERIALIZABLE — the strictest level, transactions are executed as if they were sequential. It prevents all types of anomalies but reduces concurrency.

Example in Java using Spring:

@Transactional(isolation = Isolation.REPEATABLE_READ)
public void updateData() {
    // business logic
}

The choice of isolation level depends on data consistency requirements and system performance.