如何从另一个 JOINed 表中加入 COUNT(*)

Question

我有 3 个数据库： Users 、 Posts 、 Comments 。

我想执行一个获取所有帖子信息的查询

SELECT
    Posts.id as postId,
    Users.id as authorId,
    Posts.title,
    Users.displayName,
    Posts.createdAt
FROM
    Users
INNER JOIN
    Posts
ON
    Users.id = Posts.authorId;

返回类似的东西

+--------+----------+---------+-------------+---------------------+
| postId | authorId | title   | displayName | createdAt           |
+--------+----------+---------+-------------+---------------------+
|      1 |        1 | title 1 | Alice       | 2022-07-22 16:35:39 |
|      2 |        2 | title 2 | Bob         | 2022-07-22 16:35:47 |
+--------+----------+---------+-------------+---------------------+

我想加入这个结果

SELECT postId, COUNT(*) FROM Comments GROUP BY postId

它返回类似的东西

+--------+----------+
| postId | COUNT(*) |
+--------+----------+
|      1 |        5 |
|      2 |        3 |
+--------+----------+

我想要达到的最终结果是

+--------+----------+---------+-------------+---------------------+--------------+
| postId | authorId | title   | displayName | createdAt           | commentCount |
+--------+----------+---------+-------------+---------------------+--------------+
|      1 |        1 | title 1 | Alice       | 2022-07-22 16:35:39 | 5            |
|      2 |        2 | title 2 | Bob         | 2022-07-22 16:35:47 | 3            |
+--------+----------+---------+-------------+---------------------+--------------+

我应该如何编写 SQL 查询？ （我正在使用 MySQL）

Answer 1

你有 3 张桌子（？），你可以试试这个：

SELECT
    P.ID AS POSTID,
    U.ID AS AUTHORID,
    P.TITLE,
    U.DISPLAYNAME,
    P.CREATEDAT,
    C.CNT
FROM
    USERS U
INNER JOIN
    POSTS P
ON
    U.ID = P.AUTHORID
    JOIN (SELECT POSTID, COUNT(*) CNT FROM COMMENTS GROUP BY POSTID) C
    ON C.POSTID = P.ID;

Answer 2

您可以使用子查询（“标量子查询”）来获取每个帖子的评论数。

例如：

SELECT
    p.id as postId,
    u.id as authorId,
    p.title,
    u.displayName,
    p.createdAt,
    (select count(*) from comments c where c.postid = p.id) as comment_count
FROM
    Users u
INNER JOIN 
    Posts p
ON
    u.id = p.authorId;

我在表格中添加了别名以提高可读性。

Answer 3

您可以在此处使用 CTE（通用表表达式）。 以下是查询：

with comment_count as 
(
SELECT postId, COUNT(*) as commentCount FROM Comments GROUP BY postId
),
post_processed as 
(
SELECT
    Posts.id as postId,
    Users.id as authorId,
    Posts.title,
    Users.displayName,
    Posts.createdAt
FROM
    Users,
    Posts
where
    Users.id = Posts.authorId
)
select 
pp.*,
cc.commentCount
from 
post_processed pp,
comment_count cc
where pp.postId = cc.postId;