请问一个存储过程的问题，在线马上揭题

CREATE PROCEDURE pCheck_chuqin
(@gh varchar(6),
@d_date datetime,
@a float output)
AS
SELECT gh,@a = SUM(gp_chuqin) FROM Vw_GPchuqin
WHERE (gh = @gh) AND (d_date = @d_date) GROUP BY gh

GO

为什么题示:向变量赋值的select语句不能与数据检索操作结合使用?