A classic two-bank-accounts-and-a-transfer task. If you just read both balances and update them without locks, two parallel transactions can see the same state and both deduct the money — the account ends up negative. Inside a single transaction transfer 200 from Anna's account (id 1) to Bob's (id 2): first lock both rows for the rest of the transaction in write mode, then with two updates deduct 200 from Anna and credit 200 to Bob.