Microsoft® SQL Server 2012 Unleashed

Transaction Isolation Levels in SQL Server

Isolation levels determine the extent to which data being accessed or modified in one transaction is protected from changes to the data by other transactions. In theory, each transaction should be fully isolated from other transactions. However, in practice, for practical and performance reasons, this might not always be the case. In a concurrent environment in the absence of locking and isolation, the following four scenarios can happen:

Lost update—In this scenario, no isolation is provided to a transaction from other transactions. Multiple transactions can read the same copy of data and modify it. ...