mysql - 使用 SQL 和 JOIN 进行统计投票（这可能吗？）

Question

我的问题类似于这个问题，我已经尝试过解决方案，但它并不完全适合我的场景。

我有 2 张桌子：投票和帖子。这是一个基本的草图：

`posts`
----+------------------------------------------------------------------------+
| ID | post_title                                                            |
+----+-----------------------------------------------------------------------+
|  1 | Hello world.                                                          |
|  2 | This is a post!                                                       |
|  3 | What is the meaning of life?                                          |
|  4 | Looking for a good time?                                              |
+----+-----------------------------------------------------------------------

`votes`
+----+---------+
| ID | post_id | 
+----+---------+
|  1 |     1   |  
|  2 |     1   | 
|  3 |     1   |  
|  4 |     3   | 
|  5 |     3   |  
|  6 |     4   |  
+----+---------+

问题：

我想知道每个帖子有多少票，并显示它们，使得票最高的帖子在顶部。

     Post ID   Vote Count
   +---------+-----------+
   | 1       | 3         |
   | 3       | 2         |
   | 4       | 1         |
   | 2       | 0         |

实现这一目标的 SQL 查询是什么样的？

Answer 1

select post_id, count(*)
from votes
group by post_id
order by count(*) desc

编辑：

select v.post_id, count(*)
from votes v INNER JOIN posts p ON v.post_id = p.id
group by v.post_id
order by count(*) desc

Answer 2

SELECT post_id, COUNT(*) AS tally
  FROM votes
 GROUP 
    BY post_id
UNION
SELECT ID AS post_id, 0 AS tally
  FROM posts
 WHERE ID NOT IN (SELECT post_id FROM votes);

Answer 3

如果您想在不执行 UINON 的情况下包含零票数的帖子，您可以执行

  SELECT 
         p.id, 
         SUM(CASE WHEN v.post_id IS NOT NULL THEN 1 ELSE 0 END)  AS tally
  FROM  
      posts p
      LEFT JOIN votes v
      ON v.post_id = p.id
  ORDER BY  
      SUM(CASE WHEN v.postid IS NOT NULL THEN 1 ELSE 0 END) DESC
  GROUP 
    BY p.id

此处需要 SUM/CASE，因为 COUNT(NULL) = 1

由于您的结构非常接近，因此您可以在 data.SE 上查看一个示例