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There are three typical types of inconsistency that can occur during the execution of concurrent transactions. This list is not exhaustive as other types of inconsistencies can also occur. These three types are mentioned in the ISO SQL/2008 standard and are defined in terms of the behaviors that can occur at the lower isolation levels.
Dirty read Transaction A modifies a row, but does not commit or roll back the change. Transaction B reads the modified row. Transaction A then either further changes the row before performing a COMMIT, or rolls back its modification. In either case, transaction B has seen the row in a state which was never committed.
Non-repeatable read Transaction A reads a row. Transaction B then modifies or deletes the row and performs a COMMIT. If transaction A then attempts to read the same row again, the row is changed or deleted.
Phantom row Transaction A reads a set of rows that satisfy some condition. Transaction B then executes an INSERT or an UPDATE on a row which did not previously meet A's condition. Transaction B commits these changes. These newly committed rows now satisfy Transaction A's condition. If Transaction A then repeats the read, it obtains the updated set of rows.
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