SQL SERVER 存储过程中 SQL语句无法执行成功

ALTER PROCEDURE [dbo].[T520_PACK_PASS_PRO]

-- Add the parameters for the stored procedure here

@SN varchar(20)   --这个地方不能为 @SN varchar 否则SQL 语句不会执行，这里必须指定 varchar(20) 必须有长度

AS

BEGIN

-- SET NOCOUNT ON added to prevent extra result sets from

-- interfering with SELECT statements.

SET NOCOUNT OFF;

-- Insert statements for procedure here

declare @count int

SELECT @count=count(1) from mes.dbo.jbl_label_sn where SN=@SN

return @count

END