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 -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general