The SQL standard is defined by four transaction isolation levels regarding three phenomena. The three phenomena must be prevented between concurrent transactions. The unwanted phenomenon is
Phantom read : a transaction that re-executes a query, returning a set of rows that satisfy a search condition and then finds that the location of rows that have been helping the situation has changed due to another recently committed transaction.
Non-repeatable read : a transaction that re-reads the data that it has previously read and then finds that another transaction has already modified data.
Dirty reads : a transaction that reads data written by a concurrent uncommitted transaction is the dirty read.
