下面这个函数就可以了
CREATE FUNCTION 返回月天数(@YY CHAR(4), @MM CHAR(2))
RETURNS INT
AS
BEGIN
DECLARE @Riqi DATETIME,@I INT
SET @MM=CONVERT(CHAR(2),CONVERT(INT,@MM)+1)
SET @Riqi = dateadd(day,-1,CONVERT(DATETIME,@YY+'-'+@MM+'-01'))
SET @I =CAST(datepart(day,@Riqi) AS INT)
return @I
END