如何利用SQL Server EXISTS结构

IF OBJECT_ID('SalesHistory','U')>0
      DROP TABLE SalesHistory

GO

CREATE TABLE SalesHistory

(            
      SaleID INT IDENTITY(1,1),                  
      Product VARCHAR(30),             
      SaleDate DATETIME,             
      SalePrice MONEY

)



DECLARE @i SMALLINT

SET @i = 1



WHILE (@i <= 100)



BEGIN            
      INSERT INTO SalesHistory            
      (Product, SaleDate, SalePrice)            
      VALUES            
      ('Computer', DATEADD(mm, @i, '3/10/2006'), DATEPART(ms, GETDATE()) + (@i + 57) )


      INSERT INTO SalesHistory            
      (Product, SaleDate, SalePrice)            
      VALUES            
      ('BigScreen', DATEADD(mm, @i, '3/2/2006'), DATEPART(ms, GETDATE()) + (@i + 13) )


      INSERT INTO SalesHistory            
      (Product, SaleDate, SalePrice)            
      VALUES            ('PoolTable', DATEADD(mm, @i, '2/11/2007'), DATEPART(ms, GETDATE()) + (@i + 29) )


      SET @i = @i + 1



END

GO