--?
select * from tb
where mun=1

--?
select * from tb
where mun in(select id from tb)

--?
select * from tb a
where exists(select 1 from tb where a.mun=id) and id=1

--?
select * from tb
where mun in(select id from tb) and id=1

create proc list
 @id
as
begin 
 declare @mun
 select @mun=mun from tb where id= @id
 select id from tb where mun=@mun
end

if object_id('t')is not null
   drop table t
if object_id('f')is not null
   drop function  f
go
create table t (id varchar(2) ,mun varchar(2),name varchar(5))
insert into t select '1','1','clsa'
   union all  select '2','1','clsb'
   union all  select '3','1','clsc'
   union all  select '4','2','clsd'
go
create function f (@id varchar(2))
returns @t table(id varchar(2),level int)
as 
begin
    declare @level int
    set @level=1 
    insert into @t select @id,@level
    while @@rowcount>0
    begin 
      set @level=@level + 1
      insert into @t select a.id,@level
                from t a ,@t b
                   where a.mun=b.id
                         and b.level=@level-1
    end
return
end
go
select a.* from t a ,dbo.f('1') b
      where a.id=b.id

--查找最大菜单
select * from table where num=0 -- 或者num not in(select id from table)
--查找子菜单
select * from from table where mun=@id--不就可以了吗