ACID Properties Cheat Sheet

The four ACID guarantees, SQL transaction isolation levels, and the concurrency anomalies each isolation level prevents or allows.

1 PageIntermediateMar 18, 2026

The Four Properties

What ACID actually guarantees.

Atomicity- a transaction's operations either all succeed or all roll back together
Consistency- a transaction moves the database from one valid state to another
Isolation- concurrent transactions don't observe each other's uncommitted changes
Durability- once committed, changes survive crashes, power loss, and restarts

Atomicity in Practice

A transfer that must succeed or fail as a unit.

sql

BEGIN TRANSACTION;UPDATE accounts SET balance = balance - 100 WHERE id = 1;UPDATE accounts SET balance = balance + 100 WHERE id = 2;-- if either statement fails, the whole transaction rolls backCOMMIT;-- or: ROLLBACK;

Isolation Levels (weakest to strongest)

Standard SQL isolation levels.

READ UNCOMMITTED- can see other transactions' uncommitted changes (dirty reads)
READ COMMITTED- only sees committed data; default in Postgres, Oracle, SQL Server
REPEATABLE READ- a query returns the same rows all transaction long; MySQL InnoDB default
SERIALIZABLE- transactions behave as if run one at a time; strongest, most contention

Anomalies Isolation Prevents

Concurrency problems isolation levels guard against.

Dirty read- reading another transaction's uncommitted data
Non-repeatable read- re-reading a row returns different data after another commit
Phantom read- re-running a query returns new or missing rows after another commit
Lost update- two transactions overwrite each other's changes because neither saw the other's write

Pro Tip

Durability doesn't mean the data files are written instantly — it typically means the change is recorded in a write-ahead log that's fsync'd before COMMIT returns, so crash recovery can replay it even if the data files lag behind.

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