go
create table TableA (AID varchar(2),strBID varchar(9))
insert into TableA
select 'A1',null union all
select 'A2',',B1,B3,' union all
select 'A3',',B1,B2,' union all
select 'A4',',B1,B4,'
go
create table TableB (BID varchar(2),UpdateTime datetime)
insert into TableB
select 'B1','20120130' union all
select 'B2','20120131' union all
select 'B3','20120201' union all
select 'B4','20120202'
go
create function aa(@num int)
returns varchar(100)
as
begin
    declare @sql varchar(100) set @sql=''
    --设置当前日期，正常getdate()即可
    declare @t datetime  set @t='20120203'
    select @sql=@sql+AID+',' from TableA a left join TableB b on charindex(','+BID+',',strBID)>0
    where b.UpdateTime between dateadd(d,-@num,@t) and @t
    select @sql=left(@sql,len(@sql)-1)
    return @sql
end

select dbo.aa(2)
/*
A2,A4
*/