-------建立测试环境
create table Po(id int,pup int,content varchar(10))
insert into po select 1,0,'a'
union all select 2,1,'b'
union all select 3,1,'c'
union all select 4,2,'d'
union all select 5,2,'e'
union all select 6,3,'f'
union all select 7,3,'g'
union all select 8,5,'h'
union all select 9,5,'j'
union all select 10,8,'k'
---创建函数
create function pol(@id int)
returns int
as
begin
    declare @t table(id int,pup int,lev int)
    declare @i int
    set @i=1
    insert into @t select id,pup,@i from po where id in (select pup from po where id=@id)
    while(@@rowcount>0)
    begin
        set @i=@i+1
        insert into @t select b.id,b.pup,@i from @t a,po b where a.pup=b.id and a.lev=@i-1
    end
    declare @return int
    declare @cnt int
    select @cnt=count(*) from @t
    if(@cnt>=2)
    begin
        select top 1 @return=id from (select top 2 * from @t order by lev desc)a order by lev    
    end
    else
    begin
        set @return=@id
    end
    return @return
end

------调用函数：
select dbo.pol(id),pup,content from po