Plz help me with this SQL query

alter function dbo.anf
(
@mdvn int,
@fdate datetime,
@tdate datetime
)
returns @tab table(DVN_CD int,PHC_CD int,HSC_CD int,ANC int)
AS
Begin
declare @vdate date
declare @fedddate date
declare @tedddate date
declare @tvdate date
set @vdate=dateadd(dd,-91,@fdate)
set @tvdate=dateadd(dd,36,@tdate)
set @fedddate=dateadd(dd,20,@fdate)
set @tedddate=dateadd(dd,310,@tdate)
declare @ANC int
set @ANC=ANC
select dvn_cd,phc_cd,hsc_cd,SUM(ANC) as ANC from
(
	select dvn_cd,phc_cd,hsc_cd,
	case when visit_no=3 and ANEDD between @vdate and @tvdate then 1 else 0  end as ANC 	 
	from ANVisits3 a where DVN_CD=@mdvn and ANEDD between @fedddate and @tedddate
)a group by dvn_cd,phc_cd,hsc_cd
return @tab
end