MySQL 计算两列相同且非空的地方，相同 Table

Question

我有一个 table 包含以下数据：

mysql> describe Post;
+-------------+--------------+------+-----+---------+----------------+
| Field       | Type         | Null | Key | Default | Extra          |
+-------------+--------------+------+-----+---------+----------------+
| id          | int(11)      | NO   | PRI | NULL    | auto_increment |
| user_id     | int(11)      | NO   | MUL | NULL    |                |
| post_date   | datetime     | NO   |     | NULL    |                |
| in_reply_to | int(11)      | YES  |     | NULL    |                |
| text        | varchar(160) | NO   |     | NULL    |                |
+-------------+--------------+------+-----+---------+----------------+

mysql> select id as "Row ID", user_id as "User ID", post_date as "Post Date", IF(in_reply_to is NULL, "None", in_reply_to) as "In Reply To Post ID:", CONCAT(LEFT(text,40),"...") as "Post Text" from Post;
+--------+---------+---------------------+----------------------+---------------------------------------------+
| Row ID | User ID | Post Date           | In Reply To Post ID: | Post Text                                   |
+--------+---------+---------------------+----------------------+---------------------------------------------+
|      1 |       1 | 2015-08-14 20:38:00 | None                 | This is the original test post that I pu... |
|      2 |       2 | 2015-08-14 20:39:00 | None                 | This is the second post that I put into ... |
|      3 |       5 | 2015-08-14 22:00:00 | 1                    | Hahaha, that post was hilarious. I canno... |
|      4 |       4 | 2015-08-14 23:00:00 | 1                    | Today I saw a cat jump off the roof, ont... |
|      5 |       4 | 2015-08-14 23:00:00 | None                 | Today I saw a cat jump off the roof, ont... |
|     27 |       1 | 2015-09-08 05:53:40 | 2                    | This is a mad reply ay...                   |
|     28 |       1 | 2015-09-08 11:24:05 | None                 | Yolo Swag...                                |
+--------+---------+---------------------+----------------------+---------------------------------------------+
7 rows in set (0.05 sec)

如果您不确定它们代表什么，每列都有描述。我关心这个问题的两列是 id 和 in_reply_to.

in_reply_to 是一个 NULLABLE FK 整数，在同一个 table 中引用 id；如果in_reply_to是NULL，则表示post是原始的post，如果是整数值，则表示是回复post，代表id post 的回复。

在下面的例子中有4个原始posts (1, 2, 5, 28) 和3个回复(3, 4, 27)，即3是对1的回复，4也是对 1 的回复，27 是对 2 的回复。我希望执行一个 SQL 查询，生成如下所示的输出：

intended output

其中 Num Replies 是 COUNT 表示在同一 table 中有多少行的 in_reply_to 等于 id；如果没有对该 post 的回复，则显示 0（即没有行包含特定 post 的 ID 作为其 in_reply_to 列。

谢谢。

解决方案（根据安德斯的回答）：

mysql> SELECT   Post.id, Post.user_id, Post.post_date, Post.in_reply_to, CONCAT(LEFT(Post.text,40)),   IF(counts.count IS NULL, 0, counts.count) AS 'Num of Replies' FROM Post LEFT JOIN    (SELECT      in_reply_to AS id,      COUNT(*) AS count    FROM Post    WHERE in_reply_to IS NOT NULL    GROUP BY in_reply_to) AS counts  ON Post.id = counts.id;
+----+---------+---------------------+-------------+------------------------------------------+----------------+
| id | user_id | post_date           | in_reply_to | CONCAT(LEFT(Post.text,40))               | Num of Replies |
+----+---------+---------------------+-------------+------------------------------------------+----------------+
|  1 |       1 | 2015-08-14 20:38:00 |        NULL | This is the original test post that I pu |              2 |
|  2 |       2 | 2015-08-14 20:39:00 |        NULL | This is the second post that I put into  |              1 |
|  3 |       5 | 2015-08-14 22:00:00 |           1 | Hahaha, that post was hilarious. I canno |              0 |
|  4 |       4 | 2015-08-14 23:00:00 |           1 | Today I saw a cat jump off the roof, ont |              0 |
|  5 |       4 | 2015-08-14 23:00:00 |        NULL | Today I saw a cat jump off the roof, ont |              0 |
| 27 |       1 | 2015-09-08 05:53:40 |           2 | This is a mad reply ay                   |              0 |
| 28 |       1 | 2015-09-08 11:24:05 |        NULL | Random Text                              |              0 |
+----+---------+---------------------+-------------+------------------------------------------+----------------+
7 rows in set (0.00 sec)

Answer 1

您需要在同一个 table 上加入两个查询。第一个只选择所有 post，第二个计算每个 post 的回复数。这是一个左连接，因为您想要包含 posts 而没有任何回复（不会从第二个查询返回）。 IF 用于将 NULL 值转换为 0。

SELECT
  post.id,
  -- Other fields...,
  IF(counts.count IS NULL, 0, counts.count) AS count
FROM post
LEFT JOIN 
  (SELECT
     in_reply_to AS id,
     COUNT(*) AS count
   FROM post
   WHERE in_reply_to IS NOT NULL
   GROUP BY in_reply_to) AS counts
 ON post.id = counts.id

免责声明：我没有测试过这个。

Answer 2

您可以使用传统方式进行连接，也可以直接在新列中进行连接。

示例：

select a.id, 
(select count(*) from (select 1 as id union all select 1 union all select 2)b where b.id=a.id) as count_of_replies

 from 
    (select 1 as id union all select 1  union all select 2)a

注意 2 个子查询 'tables' 都是相同的 table。

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