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What is a transaction in SQL Server, and why is grouping multiple statements into one important? How do you use a SAVE TRANSACTION savepoint within a larger transaction to allow partial rollback of just a portion of the work? What is the difference between an implicit and an explicit transaction in SQL Server? How do you check whether a transaction is currently active and view its properties within a session? How would you design a distributed transaction that needs to span operations across two different database servers, and what additional complexity does this introduce? How do long running transactions negatively impact overall system performance, and what strategies help avoid this problem?
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SQL Server › Transactions & ACID

What does ACID stand for, and what does each letter represent in the context of database transactions?

Beginner

Answer

ACID stands for Atomicity, meaning a transaction fully succeeds or fully fails with nothing in between, Consistency, meaning a transaction always leaves the database in a valid state, Isolation, meaning concurrent transactions do not interfere with each other in unexpected ways, and Durability, meaning once a transaction is committed, its changes are permanently saved even if the system crashes immediately afterward.
-- A properly committed transaction guarantees all four ACID properties
BEGIN TRANSACTION;
UPDATE Inventory SET Quantity = Quantity - 1 WHERE ProductId = 5;
COMMIT TRANSACTION; -- durably saved once committed
Real-world example A payment processing system relies on all four ACID properties together, ensuring that a completed payment transaction is fully applied, leaves account balances valid, does not interfere with other simultaneous payments, and survives even a sudden server crash immediately after committing.

Common follow-ups: Which ACID property is most directly related to how isolation levels work?;How does durability actually get guaranteed at a technical level?

Isolation & Locking;Backup & Recovery
All 7 Transactions & ACID questions