Transaction Isolation Levels

SQL Server's isolation models each attempt to conquer a subset of these problems, providing database administrators with a way to balance transaction isolation and business requirements. The five SQL Server isolation models are:

* The Read Committed Isolation Model is SQL Server’s default behavior. In this model, the database does not allow transactions to read data written to a table by an uncommitted transaction. This model protects against dirty reads, but provides no protection against phantom reads or non-repeatable reads.
* The Read Uncommitted Isolation Model offers essentially no isolation between transactions. Any transaction can read data written by an uncommitted transaction. This leaves the transactions vulnerable to dirty reads, phantom reads and non-repeatable reads.
* The Repeatable Read Isolation Model goes a step further than the Read Committed model by preventing transactions from writing data that was read by another transaction until the reading transaction completes. This isolation model protect against both dirty reads and non-repeatable reads.
* The Serializable Isolation Model uses range locks to prevent transactions from inserting or deleting rows in a range being read by another transaction. The Serializable model protects against all three concurrency problems.
* The Snapshot Isolation Model also protects against all three concurrency problems, but does so in a different manner. It provides each transaction with a "snapshot" of the data it requests. The transaction may then access that snapshot for all future references, eliminating the need to return to the source table for potentially dirty data.

If you need to change the isolation model in use by SQL Server, simply issue the command:

SET TRANSACTION ISOLATION LEVEL

where is replaced with any of the following keywords:

* READ COMMITTED
* READ UNCOMMITTED
* REPEATABLE READ
* SERIALIZABLE
* SNAPSHOT