Dirty Read

A dirty read happens when one transaction reads data that another transaction has written but not yet committed — if the writer rolls back, the reader saw data that never really existed.

Dirty reads are only possible at the loosest isolation level, Read Uncommitted (or with a hint like SQL Server's NOLOCK). Every other standard isolation level — Read Committed and above — prevents dirty reads by design, which is why Read Committed is the typical default in most databases.

It's one of the classic concurrency anomalies alongside the non-repeatable read and the phantom read, each prevented by a progressively stricter isolation level.