What transaction isolation levels are you aware of in SQL?

The main transaction isolation levels in SQL:

1. Read Uncommitted — the lowest level. A transaction can read data that has not yet been committed (dirty read).
2. Read Committed — the transaction sees only committed data. Dirty reads are prevented, but non-repeatable reads are possible.
3. Repeatable Read — guarantees that data read in a transaction will not change until the transaction ends. It prevents non-repeatable reads but allows phantom reads (new rows matching the condition may appear).
4. Serializable — the strictest level. Transactions are executed as if they were sequential, fully preventing phantom reads.

Example in PostgreSQL:

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
BEGIN;
-- operations
COMMIT;