Question

我有这个：

SELECT
   posts.id,
   (SELECT COUNT(*) FROM votes WHERE votes.post = posts.id) AS votesCount,
   (SELECT SUM(vote) FROM votes WHERE votes.post = posts.id) AS votesUp
FROM posts WHERE posts.id = 1

如何在不执行其他SELECT的情况下获得 votesCount 和 votesUp 之间的区别？类似的东西：

SELECT
   posts.id,
   (SELECT COUNT(*) FROM votes WHERE votes.post = posts.id) AS votesCount,
   (SELECT SUM(vote) FROM votes WHERE votes.post = posts.id) AS votesUp,
   votesCount - votesUp AS votesDown
FROM posts WHERE posts.id = 1

是否可能，或者我是否必须再拨打另一个SELECT？感谢。

是的，抱歉我的英语不好。

Answer 1

我建议您加入投票而不是使用子查询，然后您可以轻松地重复使用聚合：

select
  p.id,
  count(v.post) as votesCount,
  sum(v.vote) as votesUp,
  count(v.post) - sum(v.vote) as votesDown
from
  posts p
  left join votes v on v.post = p.id
where
  p.id = 1
group by
  p.id

Answer 2

您的子查询不太有意义。通常，您希望使用子查询作为最后的手段，而不是连接。例如：

SELECT
    posts.id, COUNT(votes.*) AS votesCount, SUM(vote) AS votesUp 
FROM posts
    LEFT JOIN votes ON votes.post = posts.id
WHERE
    posts.id = 1
GROUP BY posts.id

这也使得减法更容易，但遗憾的是它需要一些重复（别名不能用于后续列：/）：

SELECT
    posts.id, COUNT(votes.*) AS votesCount, SUM(vote) AS votesUp,
    COUNT(votes.id) - SUM(vote) AS votesDown
FROM posts
    LEFT JOIN votes ON votes.post = posts.id
WHERE
    posts.id = 1
GROUP BY posts.id

Answer 3

甚至没有另一个嵌套的SELECT？

    SELECT
    posts.id,
    (SELECT COUNT(*) FROM votes WHERE votes.post = posts.id) AS votesCount,
    (SELECT SUM(vote) FROM votes WHERE votes.post = posts.id) AS votesUp
    (SELECT SUM(CASE WHEN vote=0 THEN 1 ELSE 0 END) FROM votes WHERE votes.post = posts.id) AS votesDown
    FROM posts WHERE posts.id = 1

多次使用'as'

3 个答案: