DISTINCT is really different from DISTINCT ON. And to be honest you sound as if you do not realize that.

DISTINCT is a general SQL feature that all RDBMS have.

DISTINCT ON is a PostgreSQL-only extension. See my examples, and the SELECT manual page that I linked to upthread.

So I still think that the last statement in my example (the statement that reduces your 20-row set to a 13-row set) does what you require. Is that not so? 

select

  -- this line is the DISTINCT ON clause:
     distinct on (key)

  -- the select list follows:
     key,
     source

from yourtable

-- often you'd want an order by clause
-- to control which rows are discarded
-- In this case it does not seem to be important
;

[download]

In reply to Re^3: Contemplating some set comparison tasks by erix
in thread Contemplating some set comparison tasks by dwhite20899

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.