Re: Query m:n-Combination

Ludwig Kniprath wrote:
> A typical M:N-constellation, rivers in one table, communities in the
> other table, m:n-join-informations (which river is running in which
> community) in a third table.
>
> Table rivers:
> R_ID R_Name
> 1 river_1
> 2 river_2
> 3 river_3
> 4 river_4
> 5 river_5
>
> Table communities :
> C_ID C_Name
> 1 community_1
> 2 community_2
> 3 community_3
> 4 community_4
> 5 community_5
>
> Join-table
> mn_2_r_id mn_2_c_id
> 1 1
> 1 2
> 1 3
> 1 4
> 2 1
> 3 2
> 3 5
> 4 3
> ...
>
> I want to know, which river is running through communities
> 1,2,3 *and* 4?
> You can see the solution by just looking at the data above (only
> "river_1" is running through all these countries), but how to
> query this by sql?

SELECT r.r_name FROM rivers AS r
JOIN join-table j1 ON (r.r_id = j1.mn_2_r_id)
JOIN join-table j2 ON (r.r_id = j2.mn_2_r_id)
JOIN join-table j3 ON (r.r_id = j3.mn_2_r_id)
JOIN join-table j4 ON (r.r_id = j4.mn_2_r_id)
WHERE j1.mn_2_c_id = (SELECT c_id FROM communities WHERE c_name = 'community_1')
AND j2.mn_2_c_id = (SELECT c_id FROM communities WHERE c_name = 'community_2')
AND j3.mn_2_c_id = (SELECT c_id FROM communities WHERE c_name = 'community_3')
AND j4.mn_2_c_id = (SELECT c_id FROM communities WHERE c_name = 'community_4')

(untested)

Is that what you are looking for?

Yours,
Laurenz Albe