On Thu, 2008-10-09 at 10:59 -0600, Bill Thoen wrote: > >> I'm trying to combine two tables, but I only want unique records based > >> on the first two columns. Can UNION be used to join three-column tables > >> but only include records based on the uniqueness of the first two > >> columns? If not, how would I do this with PostgreSQL 8.1? > > > > How do you decide which records you want? - e.g. given the following rows... > > > > (a, b, c) > > (a, b, d) > > > > ...how do you decide whether you the one with c or the one with d? > > > > > The physical order that they appear will take care of that. If the rest of the columns don't matter, how about: SELECT DISTINCT ON(col1, col2) * FROM ( SELECT col1, col2, col3 FROM table1 UNION SELECT col1, col2, col3 FROM table2 ORDER BY col1, col2 ) AS uniontable; - Josh Williams
