MySQL begränsar resultat per kategori

MySQL har ingen rankningsfunktion, men du kan använda en variabel för att skapa ett pseudo-radnummer.

Använd:

SELECT x.*
  FROM (SELECT cm.id,
               cm.title AS cmtitle,
               cm.sectionid,
               cm.type AS cmtype,
               cd.id AS cd_id,
               cd.time,
               cd.link,
               cd.title,
               cd.description,
               cd.sectionid AS cd_sectionid,
               CASE
                 WHEN @sectionid != cm.sectionid THEN @rownum := 1 
                 ELSE @rownum := @rownum + 1
               END AS rank,
               @sectionid := cm.sectionid
          FROM C_MAIN cm,
               C_DATA cd,
               (SELECT @rownum := 0, @sectionid := NULL) r
         WHERE cm.sectionid = cd.sectionid
      ORDER BY cm.sectionid) x
 WHERE x.rank <= 20
ORDER BY id