Glossary

Transaction (DB)

A transaction is a unit of database work that either completes in full or not at all. If an error occurs during a money transfer between two accounts — after the debit but before the credit — the transaction rolls back and balances remain unchanged.

ACID properties

Atomicity — all operations succeed or all are rolled back
Consistency — the database moves from one valid state to another
Isolation — concurrent transactions cannot see each other's uncommitted changes
Durability — after COMMIT, changes are persisted even on failure

SQL syntax

BEGIN;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
UPDATE accounts SET balance = balance + 100 WHERE id = 2;
COMMIT;

If an error occurs between BEGIN and COMMIT — issue a ROLLBACK to undo all changes. Most drivers and ORMs wrap this automatically.