acid sql is the four-letter contract — Atomicity , Consistency , Isolation , Durability — that every relational database honours the moment you wrap statements in BEGIN … COMMIT . Knowing the contract is table stakes. Knowing how each letter is implemented in production SQL — WAL and fsync for Durability , CHECK / FOREIGN KEY / UNIQUE constraints for Consistency , SET TRANSACTION ISOLATION LEVEL for Isolation , ROLLBACK for Atomicity — and how it trades against base properties and the cap theorem when the workload goes global, is the senior data-engineering interview signal panelists actually score on.…