--> 测试数据: @表A
declare @表A table (ID int,Name varchar(1))
insert into @表A
select 20,'a' union all
select 120,'b' union all
select 30,'c' union all
select 41,'d' union all
select 51,'e'

--> 测试数据: @表B
declare @表B table (Qu varchar(2),IDN varchar(10))
insert into @表B
select 'la','41' union all
select 'lb','30,20,120'

select a.*,b.Qu from @表A a 
left join @表B b on charindex(','+ltrim(a.ID)+',',','+b.IDN+',')>0

/*
ID          Name Qu
----------- ---- ----
20          a    lb
120         b    lb
30          c    lb
41          d    la
51          e    NULL
*/