如何获得每组的最大行数？

Question

我有一个数据库，人们可以在其中为不同地方的人提交选票。在给定的时间，我想找出每个地方谁的票数最多。 （一个人可以在两个不同的地方投票）

这是我目前的 SQL：

SELECT placeId, userVotedId, cnt 
FROM 
    (SELECT uvo.userVotedId, p.placeId, count(*) as cnt 
     FROM users as u, users_votes as uvo, places as p 
     WHERE u.userId = uvo.userVotedId 
       AND p.placeId = uvo.placeId 
     GROUP BY userVotedId, placeId) 
AS RESULT

这给了我这个结果：

现在，这些是我真正想要的行：

我的查询中缺少什么，所以我可以得到这个？

• 我希望每个位置有一个结果。所以我应该只看到不同的 placeId，其中 userVotedId 获得了最多的选票。

• 如果出现平局，则随机获胜！

Answer 1

看来你还需要一个聚合体。在您的 cnt 值和 GROUP BY placeId, userVotedId 上使用 MAX() 聚合：

SELECT placeId, userVotedId, max(cnt)
FROM 
(
  SELECT uvo.userVotedId, p.placeId, count(*) as cnt 
  FROM users as u
  INNER JOIN users_votes as uvo
    ON u.userId = uvo.userVotedId 
  INNER JOIN places as p 
    ON p.placeId = uvo.placeId 
  GROUP BY userVotedId, placeId
) AS RESULT
GROUP BY placeId, userVotedId

注意：我将您的查询更改为使用 JOIN 语法，而不是表之间的逗号。

编辑，根据您的评论，以下应该有效：

select total.uservotedid,
  total.placeid,
  total.cnt
from
(
  SELECT uvo.userVotedId, p.placeId, count(*) as cnt 
  FROM users as u
  INNER JOIN users_votes as uvo
    ON u.userId = uvo.userVotedId 
  INNER JOIN places as p 
    ON p.placeId = uvo.placeId 
  GROUP BY userVotedId, placeId
) total
inner join
(
  select max(cnt) Mx, placeid
  from
  (
    SELECT uvo.userVotedId, p.placeId, count(*) as cnt 
    FROM users as u
    INNER JOIN users_votes as uvo
      ON u.userId = uvo.userVotedId 
    INNER JOIN places as p 
      ON p.placeId = uvo.placeId 
    GROUP BY userVotedId, placeId
  ) mx
  group by placeid
) src
  on total.placeid = src.placeid
  and total.cnt = src.mx

见SQL Fiddle with Demo

结果是：

| USERVOTEDID | PLACEID | CNT |
-------------------------------
|          65 |      11 |   1 |
|          67 |      13 |   1 |
|          67 |      25 |   1 |
|          67 |      51 |   2 |

编辑#2，如果你想在平局时返回一个随机数，那么你可以使用用户变量：

select uservotedid,
  placeid, 
  cnt
from
(
  select total.uservotedid,
    total.placeid,
    total.cnt,
    @rownum := case when @prev = total.placeid then @rownum+1 else 1 end rownum,
    @prev := total.placeid pplaceid
  from
  (
    SELECT uvo.userVotedId, p.placeId, count(*) as cnt 
    FROM users as u
    INNER JOIN users_votes as uvo
      ON u.userId = uvo.userVotedId 
    INNER JOIN places as p 
      ON p.placeId = uvo.placeId 
    GROUP BY userVotedId, placeId
  ) total
  inner join
  (
    select max(cnt) Mx, placeid
    from
    (
      SELECT uvo.userVotedId, p.placeId, count(*) as cnt 
      FROM users as u
      INNER JOIN users_votes as uvo
        ON u.userId = uvo.userVotedId 
      INNER JOIN places as p 
        ON p.placeId = uvo.placeId 
      GROUP BY userVotedId, placeId
    ) mx
    group by placeid
  ) src
    on total.placeid = src.placeid
    and total.cnt = src.mx
  order by total.placeid, total.uservotedid
) src
where rownum = 1
order by placeid, uservotedid

见SQL Fiddle with Demo

Answer 2

SELECT placeId, userVotedId, MAX(cnt)
    FROM (SELECT uvo.userVotedId, p.placeId, count(*) AS cnt 
          FROM users as u, users_votes as uvo, places as p 
          WHERE u.userId = uvo.userVotedId AND p.placeId = uvo.placeId 
          GROUP BY userVotedId, placeId) AS RESULT
    GROUP BY  placeId

• 类似问题 - SQL query max(), count()

• Demo

Answer 3

为简单起见，我将您的查询称为测试：

SELECT * 
FROM Test T JOIN (
SELECT t.placeId, Max(t.cnt) maxcnt
FROM Test t
GROUP BY t.placeId) T2 ON T.placeId = T2.placeId and T.cnt = T2.maxcnt

这里是Fiddle。

顺便说一句——测试=：

SELECT uvo.userVotedId, p.placeId, count(*) as cnt 
  FROM users as u
  INNER JOIN users_votes as uvo
    ON u.userId = uvo.userVotedId 
  INNER JOIN places as p 
    ON p.placeId = uvo.placeId 
  GROUP BY userVotedId, placeId

祝你好运。

--EDIT -- 按照要求，这是最终代码：

SELECT * 
FROM (SELECT uvo.userVotedId, p.placeId, count(*) AS cnt 
          FROM users as u, users_votes as uvo, places as p 
          WHERE u.userId = uvo.userVotedId AND p.placeId = uvo.placeId 
          GROUP BY userVotedId, placeId) T JOIN (
SELECT t.placeId, Max(t.cnt) maxcnt
FROM (SELECT uvo.userVotedId, p.placeId, count(*) AS cnt 
          FROM users as u, users_votes as uvo, places as p 
          WHERE u.userId = uvo.userVotedId AND p.placeId = uvo.placeId 
          GROUP BY userVotedId, placeId) t
GROUP BY t.placeId) T2 ON T.placeId = T2.placeId and T.cnt = T2.maxcnt