[Old Pedant]

I think this is *close* to what you want:

SELECT webName, value AS titleSort, value AS title, pageID, X.catID, pageName, sortBy 
FROM pages, pagesCats, pagesIndex,
    ( SELECT catID, MIN(sortBy) AS minsort 
      FROM pagesIndex,pages 
      WHERE pages.ID=pagesIndex.pageID GROUP BY catID ) AS X
WHERE pagesIndex.catID = X.catID
  AND pagesindex.pageID = pages.ID
  AND pages.sortBy = X.minsort
  AND pagesCats.ID = X.catID
  AND subsetof = '2'
ORDER BY X.minsort, pageID

But it does get duplicate catID/sortBy values. I'm not sure if you want that or not.