A transaction reads the same row twice but gets different values. Which anomaly has occurred?
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Non-repeatable read — a transaction reads the same row twice and gets different committed values because another transaction updated and committed that row between the two reads.
Direct answer
This is a non-repeatable read. It occurs when a transaction reads the same row twice and gets different committed values because another transaction updated that row and committed between the two reads.
Example
T1 reads an employee salary as 50,000. T2 updates that same salary to 60,000 and commits. When T1 reads the row again, it now sees 60,000. The same query against the same row has produced two different values during one transaction.
- Scenario: T1 reads 50,000 → T2 updates to 60,000 and commits → T1 reads 60,000
How this differs from other anomalies
This is different from a dirty read because the second value is committed. It is also different from a phantom read, which normally concerns rows being added or removed from a result set that matches a condition.
Mitigation / isolation
Repeatable Read or a stronger isolation level is generally used when the same row must remain stable for repeated reads within a transaction, although exact behavior depends on the DBMS implementation.