Transaction Isolation Levels
SQL Server supports four TILs. As mentioned earlier, a transaction's isolation level controls how it impacts and is impacted by other transactions. The trade-off is always one of data consistency versus concurrency. Selecting a more restrictive TIL increases data consistency at the expense of accessibility. Selecting a less restrictive TIL increases concurrency at the expense of data consistency. The trick is to balance these opposing interests so that the needs of your application are met.
Use the SET TRANSACTION ISOLATION LEVEL command to set a transaction's isolation level. Valid TILs include READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, and SERIALIZABLE.
Specifying READ UNCOMMITTED is essentially ...