获取某月最后一天

例：获取12月的第一天和最后一天

set  @bdate = '2020-12-23'
SELECT @fdate=DATEADD(MM,DATEDIFF(MM,0,@bdate),0),@ldate = DATEADD(MS,-3,DATEADD(MM, DATEDIFF(m,0,@bdate)+1, 0))