sql语句case的用法

create function [dbo].[dhp](@stcd varchar(10),@d1 varchar(100) )-- stcd 表示站点编号 d1表示时间
returns decimal
as
begin
declare @dhp decimal
if (datename(hour,@d1)>'8')
select @dhp= sum(drp) from st_pptn_r where stcd=@stcd and tm between @d1
and convert(varchar(100),year(dateadd(day,1,@d1)))
+'-'+convert(varchar(100),month(dateadd(day,1,@d1)))
+'-'+convert(varchar(100),day(dateadd(day,1,@d1)))
+' 8:00:00'
else
select @dhp= sum(drp) from st_pptn_r where stcd=@stcd
and tm between convert(varchar(100),year(dateadd(day,-1,@d1)))
+'-'+convert(varchar(100),month(dateadd(day,-1,@d1)))
+'-'+convert(varchar(100),day(dateadd(day,-1,@d1)))
+' 9:00:00' and @d1
return @dhp
end

怎么把函数转化为sql语句！急需指点！

select case when stcd='stcd' and tm between 'd1' and convert(varchar(100),year(dateadd(day,1,'d1')))
+'-'+convert(varchar(100),month(dateadd(day,1,'d1')))
+'-'+convert(varchar(100),day(dateadd(day,1,'d1')))
+' 8:00:00' then sum(drp) when stcd='stcd' and tm between convert(varchar(100),year(dateadd(day,-1,'d1')))
+'-'+convert(varchar(100),month(dateadd(day,-1,'d1')))
+'-'+convert(varchar(100),day(dateadd(day,-1,'d1')))
+' 9:00:00' and 'd1' then sum(drp) end drpsum from st_pptn_r

你把'stcd'和'd1'换成具体值，试试看。