[SQL] NOLOCK vs SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED

http://msdn.microsoft.com/en-us/library/aa259216(SQL.80).aspx

 

[READ UNCOMMITTED]

Implements dirty read, or isolation level 0 locking, which means that no shared locks are issued and no exclusive locks are honored.

When this option is set, it is possible to read uncommitted or dirty data; values in the data can be changed and rows can appear or disappear in the data set before the end of the transaction.

This option has the same effect as setting NOLOCK on all tables in all SELECT statements in a transaction.

This is the least restrictive of the four isolation levels.

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