SQL Transaction Isolation Levels:

In Simple Words: Controls how visible uncommitted changes from other transactions are:

1. Read Uncommitted: Lowest isolation; allows dirty reads.
2. Read Committed (Default in Postgres): Reads only committed rows.
3. Repeatable Read (Default in MySQL InnoDB): Guarantees the same row read twice inside a transaction returns identical data.
4. Serializable: Highest isolation; transactions execute as if run strictly one after another.

-- Setting isolation level for a sensitive financial transaction:
BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
SELECT balance FROM accounts WHERE id = 42;
UPDATE accounts SET balance = balance - 500 WHERE id = 42;
COMMIT;