Use the TOP clause to restrict the data _mssql UPDATE statement updates

You can use the TOP clause to limit the number of rows modified in the UPDATE statement. When the top (n) clause is used with UPDATE, the delete operation is performed on the randomly selected rows of N. For example, suppose you want to reduce the sales burden for a senior salesperson and assign some customers to a junior salesperson. The following example assigns 10 customers who are randomly sampled from one salesperson to another.

 Use ADVENTUREWORKS2008R2;
 UPDATE top Sales.Store
 SET SalesPersonID = 276
 WHERE SalesPersonID = 275;
 Go

If you need to use top to apply updates that are in a meaningful chronological order, you must use both the top and the ordered by clauses. The following example updates the number of vacation hours for the first 10 employees hired.

 UPDATE humanresources.employee
 SET vacationhours = vacationhours + 8 from
 (SELECT up BusinessEntityID from Hum Anresources.employee ORDER by
   hiredate ASC) as th
 WHERE HumanResources.Employee.BusinessEntityID = th. BusinessEntityID;
 Go

Reference: http://technet.microsoft.com/zh-CN/library/ms180971