When a transaction retrieves data that another transaction might have updated.
Dirty reads
脏读 | 无效数据的读出 | uncommitted dependency
When a transaction retrieves a row that has been updated by another transaction that is not yet committed.
Non-repeatable reads
不可重复读
When a transaction retrieves a row twice and that row is updated by another transaction that is committed in between.
Phantom reads
幻读
When a transaction retrieves a set of rows twice and new rows are inserted into or removed from that set by another transaction that is committed in between.