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Data & Analytics · Intermediate

Dirty Read

Reading data written by another transaction that has not yet committed and may be rolled back.

Definition

Dirty reads can produce results based on data that never officially existed. Only the Read Uncommitted isolation level, or SQL Server's NOLOCK hint, allows them.

Example

Report reads a price changed by an uncommitted transaction that is later rolled back.

Exam tip

NOLOCK equals Read Uncommitted and allows dirty reads; it is not a free performance fix.

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