Database Transactions and ACID Properties

What is a Transaction?

A transaction is a group of operations treated as a single unit — either all succeed or none do. Essential for things like money transfers.

The Classic Example

BEGIN;
  UPDATE accounts SET balance = balance - 100 WHERE id = 1;
  UPDATE accounts SET balance = balance + 100 WHERE id = 2;
COMMIT;  -- both happen, or ROLLBACK undoes everything

ACID Properties

Property	Meaning
Atomicity	All or nothing
Consistency	Valid state to valid state
Isolation	Concurrent transactions don't interfere
Durability	Committed data survives crashes

Rolling Back

BEGIN;
  UPDATE accounts SET balance = balance - 100 WHERE id = 1;
  -- error detected!
ROLLBACK;  -- undo everything

FAQs

Do NoSQL databases support transactions?

Many now do (MongoDB added multi-document transactions). More in our Database guides.

What are isolation levels?

Settings (Read Committed, Serializable...) that control how transactions see each other's changes.