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Re: Query m:n-Combination

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In article <4901993F.9000401@xxxxxxxxxxxxx>,
Ludwig Kniprath <ludwig@xxxxxxxxxxxxx> writes:

> Dear list,
> I have to solve a simple Problem, explained below with some sample-Data.

> 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
> ...

> (in real database this relation is an gis-relation with thousands of
> rivers and countries, related by spatial join, but the problem is the
> same...)

> 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?

Probably the fastest way is to do an OR join and counting the matches:

  SELECT r.r_name
  FROM rivers r
  JOIN join_table j ON j.mn2_r_id = r.r_id
  JOIN communities c ON c.c_id = j.mn2_c_id
  WHERE c.c_name IN ('community_1', 'community_2',
                     'community_3', 'community_4')
  GROUP BY r.r_name
  HAVING count(*) = 4


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