Get n grouped categories and sum others into one

Question

I have a table with the following structure:

Contents (
  id
  name
  desc
  tdate
  categoryid
  ...
)
         


        

Answer 1

You can approach this with nested aggregation. The inner aggregation calculates the counts along with a sequential number. You want to take everything whose number is 7 or less and then combine everything else into the others category:

SELECT (case when seqnum <= 7 then label else 'others' end) as label,
       (case when seqnum <= 7 then catid end) as catid, sum(cnt)
FROM (SELECT ca.name AS label, ca.id AS catid, COUNT(c.id) AS cnt,
             row_number() over (partition by ca.name, catid order by count(c.id) desc) as seqnum
      FROM contents c LEFT OUTER JOIN
           category ca
           ON ca.id = c.categoryid
      GROUP BY label, catid
     ) t
GROUP BY (case when seqnum <= 7 then label else 'others' end),
         (case when seqnum <= 7 then catid end) 
ORDER BY cnt DESC ;