Question

我想知道如何根据MySQL中的字段值返回基于count的数字。

所以，让我们说你有一个简单的MySQL设置，如：

Users (id)
Posts (id, user_id, body)
Votes (id, post_id, user_id, vote_type)

然后你想得到前10个帖子，按照vote_type='up'比vote_type='down'

多少条记录排序

SELECT votes.*, ((SELECT count(post_id) FROM votes AS v1 WHERE vote='up' 
AND votes.post_id =v1.post_id) - (SELECT count(post_id) FROM votes AS v2 
WHERE vote='down' AND votes.post_id =v2.post_id)) AS score FROM votes GROUP 
BY votes.post_id ORDER BY score DESC

以上查询有效，但不会考虑没有投票的帖子，而且似乎还有更好的方法。

Answer 1

试试这个

SELECT
  posts.*, (a.up_count - b.down_count) as net_vote
FROM
  posts
INNER JOIN (
  SELECT
    post_id, COUNT(*) as up_count
  FROM
    votes
  WHERE
    vote_type = 'up'
  GROUP BY
    post_id
) a ON a.post_id = posts.id
INNER JOIN (
  SELECT
    post_id, COUNT(*) as down_count
  FROM
    votes
  WHERE
    vote_type = 'down'
  GROUP BY
    post_id
) b ON b.post_id = posts.id
ORDER BY
  net_vote DESC
LIMIT 10

Answer 2

你可以使用一些连接

    SELECT votes.*, ifnull(v1.count,0) - innull(v2.count,0) AS score
    FROM votes
    LEFT  JOIN ( 
        SELECT post_id, count(post_id) as count
        FROM votes AS v1 WHERE vote='up' 
        GROUP BY post_id ) v1 on votes.post_id = v1.post_id
    LEFT JOIN (
        SELECT post_id, count(post_id) 
        FROM votes AS v1 WHERE vote='down' 
        GROUP BY post_id ) v2  on votes.post_id = v2.post_id

Answer 3

试试这个：

select p.*
from posts p
left join votes v on v.post_id = p.id
group by 1, 2, 3
order by sum(vote_type = 'down') - sum(vote_type = 'up')
limit 10

注意计算扭曲次数的简单方法;在MySQL中，true为1，false为0，因此我们可以对条件求和。

如果您还想查看投票类型计数的差异，请按表达式将订单添加到选择列表中。

如何基于mysql中的简单方程进行计数

3 个答案: