What are ACID properties in databases?
Why Interviewers Ask This
Interviewers use this question to quickly assess whether a candidate has the foundational knowledge required for MySQL / SQL development. It reveals whether you understand the building blocks that more complex concepts rely on.
Answer
ACID is an acronym for the four properties that guarantee reliable database transactions: Atomicity: A transaction is treated as a single indivisible unit — either ALL changes succeed (COMMIT) or NONE are applied (ROLLBACK). There is no partial transaction. If step 3 of a 5-step transaction fails, steps 1 and 2 are automatically undone. Consistency: A transaction brings the database from one valid state to another valid state, honoring all defined rules (constraints, triggers, cascades). A transaction cannot violate database integrity constraints. Isolation: Concurrent transactions execute independently — one transaction's intermediate state is not visible to other transactions. The degree of isolation is configurable (isolation levels). Even if 100 transactions run simultaneously, each behaves as if it were the only transaction. Durability: Once a transaction is COMMITTED, it is permanently saved — even if the system crashes immediately after. Committed data survives server failures, power outages, and crashes (achieved through write-ahead logging/WAL). ACID compliance is a hallmark of relational databases like MySQL InnoDB, PostgreSQL, and Oracle.
Common Mistake
Candidates often give textbook answers here. Interviewers are more impressed when you relate the concept to a specific problem you solved in a real MySQL / SQL project.