Sådan bestiller du forskellige tuples i en PostgreSQL-forespørgsel

Længst til venstre ORDER BY elementer kan ikke være uenige med elementerne i DISTINCT klausul. Jeg citerer manualen om DISTINCT kode> :

Prøv:

SELECT *
FROM  (
    SELECT DISTINCT ON (c.cluster_id, feed_id) 
           c.cluster_id, num_docs, feed_id, url_time 
    FROM   url_info u
    JOIN   cluster_info c ON (c.cluster_id = u.cluster_id) 
    WHERE  feed_id IN (SELECT pot_seeder FROM potentials) 
    AND    num_docs > 5
    AND    url_time > '2012-04-16'
    ORDER  BY c.cluster_id, feed_id, num_docs, url_time
           -- first columns match DISTINCT
           -- the rest to pick certain values for dupes
           -- or did you want to pick random values for dupes?
    ) x
ORDER  BY num_docs DESC;

Eller brug GROUP BY :

SELECT c.cluster_id
     , num_docs
     , feed_id
     , url_time 
FROM   url_info u
JOIN   cluster_info c ON (c.cluster_id = u.cluster_id) 
WHERE  feed_id IN (SELECT pot_seeder FROM potentials) 
AND    num_docs > 5
AND    url_time > '2012-04-16'
GROUP  BY c.cluster_id, feed_id 
ORDER  BY num_docs DESC;

Hvis c.cluster_id, feed_id er de primære nøglekolonner for alle (begge i dette tilfælde) tabeller, som du inkluderer kolonner fra i SELECT liste, så virker dette bare med PostgreSQL 9.1 eller senere.

Ellers skal du GROUP BY resten af ​​kolonnerne eller samle eller give flere oplysninger.