如何巧妙利用SQL Server的EXISTS结构

IF EXISTS

(

      SELECT * FROM SalesHistory

      WHERE SaleDate = '1/1/2001' AND

            Product = 'Computer' AND 

            SalePrice = 1000

)

      UPDATE TOP(1) SalesHistory

      SET SalePrice = 1100

      WHERE SaleDate = '1/1/2001' AND

            Product = 'Computer' AND 

            SalePrice = 1000
ELSE
      INSERT INTO SalesHistory

      (Product, SaleDate, SalePrice)

      VALUES

      ('Computer', '1/1/2001', 1100)