A transaction groups several statements into one unit of work that either fully happens or not at all. It is what makes a money transfer safe.

ACID

Property	Guarantee	Mechanism
Atomicity	All statements succeed, or none take effect	Undo/rollback log
Consistency	Each transaction moves the DB from one valid state to another (constraints hold)	Constraints, triggers, your logic
Isolation	Concurrent transactions don't see each other's partial work (to a chosen degree)	Locks and/or MVCC snapshots
Durability	Once committed, changes survive crashes and power loss	Write-ahead log (WAL) flushed to disk

BEGIN, COMMIT, ROLLBACK

SQL

BEGIN;
UPDATE accounts SET balance = balance - 200 WHERE id = 1;
UPDATE accounts SET balance = balance + 200 WHERE id = 2;
COMMIT;

SELECT * FROM accounts ORDER BY id;

Output

id | owner | balance
---+-------+--------
1  | Asha  | 800
2  | Ravi  | 700

SQL

BEGIN;
UPDATE accounts SET balance = balance - 200 WHERE id = 1;
-- …the app detects a problem before crediting Ravi
ROLLBACK;

SELECT * FROM accounts ORDER BY id;

Output

id | owner | balance
---+-------+--------
1  | Asha  | 1000
2  | Ravi  | 500

When a statement fails

A transfer of 1500 breaks the CHECK (balance >= 0) constraint on the debit:

SQL

BEGIN;
UPDATE accounts SET balance = balance + 1500 WHERE id = 2;   -- credit first
UPDATE accounts SET balance = balance - 1500 WHERE id = 1;   -- fails
COMMIT;

Error

CHECK constraint failed: balance >= 0

Savepoints: partial rollback

SQL

BEGIN;
INSERT INTO accounts VALUES (3, 'Meera', 300);
SAVEPOINT before_bonus;
UPDATE accounts SET balance = balance + 10000 WHERE id = 3;   -- mistake
ROLLBACK TO before_bonus;                                     -- undo only the bonus
COMMIT;

SELECT * FROM accounts ORDER BY id;

Output

id | owner | balance
---+-------+--------
1  | Asha  | 1000
2  | Ravi  | 500
3  | Meera | 300

Autocommit

Outside an explicit BEGIN, most engines run in autocommit mode: every statement is its own transaction. Oracle is the exception — a transaction starts implicitly and you must COMMIT. Python's DB-API and many drivers open transactions for you and require conn.commit().

Explain ACID with a bank transfer.

What is the difference between COMMIT and ROLLBACK?