What is a transaction?
A transaction is a series of operations (SQL statements) on the database with the following characteristics:
Atomicity The effects of a transaction are either realized completely or not at all.
Consistency In terms of application logic, the database is in a consistent state both prior to and after a transaction.
Isolation Concurrent transactions are performed in such a way that they do not affect each other at all or only to the degree defined by the isolation level.
Permanence The effects of a transaction are lasting; i.e., they survive both the end of the session and the end of database operation and are protected against software and hardware failures.