What are the typical anomalies at different transaction isolation levels?

Typical anomalies at various transaction isolation levels:

1. Read Uncommitted

   • Dirty Read — a transaction reads data that has been modified but not yet committed by another transaction.
   • Non-repeatable Read — data may change if read again within the same transaction.
   • Phantom Read — new rows matching the condition may appear upon re-query.

2. Read Committed

   • Non-repeatable Read — data can change between two reads within the same transaction.
   • Phantom Read — new rows may appear.
   • Dirty Read is excluded.

3. Repeatable Read

   • Phantom Read — possible, as new rows can appear, although already read values do not change.
   • Dirty Read and Non-repeatable Read are excluded.

4. Serializable

   • Eliminates all the above anomalies, transactions are executed as if they were sequential.

These anomalies affect data correctness and consistency, so the choice of isolation level depends on application requirements and a trade-off between performance and consistency.