MySQL: how to get x number of results per grouping [duplicate]

问题

Possible Duplicate:
mysql: Using LIMIT within GROUP BY to get N results per group?

I have a two tables:

Items
Categories

Each item belongs to a category. What I want to do is select 5 items per category but say 20 items in total.

SELECT 

   item_id, item_name, items.catid 

FROM

   items, categories

WHERE

   items.catid = categories.catid

GROUP BY items.catid LIMIT 0,5 //5 per category group

Edit: if there are more than 5 items per category - they should be ordered by the item_id (numeric value)

回答1:

Try this query -

SELECT item_id, item_name, catid FROM 
  (SELECT t1.*, COUNT(*) cnt FROM items t1
    LEFT JOIN items t2
      ON t2.catid = t1.catid AND t2.item_id <= t1.item_id 
  GROUP BY
    t1.catid, t1.item_id
  ) t
WHERE
  cnt < 6
-- LIMIT 20

It will show first 5 items per category. Uncomment LIMIT 20 if you need. Join Categories table if you need.

回答2:

SELECT item_id, item_name, catid
FROM (  SELECT item_id, item_name, catid,
            CASE WHEN @catid != catid THEN (@rownum := 1) + !(@catid := catid) ELSE @rownum := $rownum + 1 AS rownum
        FROM (  SELECT item_id, item_name, i.catid
                FROM items AS i
                INNER JOIN categories AS c
                ON i.catid = c.catid
                ORDER BY i.catid ASC) AS h, (SELECT @catid := NULL, @rownum := NULL) AS var
                HAVING rownum <= 5
                LIMIT 20) AS h2
ORDER BY item_id

Devart's solution looks right, but I have a bit of a pickle understanding it fully. Mine may not make much more sence to others, but it does to me.

EDIT: Understand Devart's solution now, and it is quite brilliant, actually. :)

来源：https://stackoverflow.com/questions/13359640/mysql-how-to-get-x-number-of-results-per-grouping

标签

mysql

sql

group-by

greatest-n-per-group