The normal clause would look like:
SELECT c.companyname, s.sectorname FROM company c, sector s, company_sector cs WHERE cs.cid = c.cid AND cs.sid = s.sid ORDER BY c.companyname;
c.companyname | ??? (sectors) ---------------+------------------------ company1 | sectora, sectorb company2 | sectora, sectorb, sectorc company3 | sectora company4 | sectorc
Search the mailing-list archives for "custom aggregate concat" and you'll quickly find an example of how to write your own custom aggregate (like SUM()).
Warning - I don't think you can guarantee the order of elements in the aggregated sectors.
-- Richard Huxton Archonet Ltd
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