如何使用Join编写等效的SQL查询？

shibly 发表于 Dev

悄悄地

原始SQL查询：

SELECT *
FROM
  (SELECT p.id,
          p.title,
          p.mark,

     (SELECT max(created)
      FROM comments c
      WHERE c.post_id=p.id
        AND c.mark=1) AS latest_at
   FROM posts p) AS Post
WHERE Post.latest_at IS NOT NULL
ORDER BY latest_at DESC LIMIT 10

我正在尝试使用join编写等效的sql查询。我该怎么做？

mysql> describe posts;
+-------------+--------------+------+-----+---------+----------------+
| Field       | Type         | Null | Key | Default | Extra          |
+-------------+--------------+------+-----+---------+----------------+
| id          | int(11)      | NO   | PRI | NULL    | auto_increment |
| user_id     | int(11)      | NO   |     | NULL    |                |
| title       | varchar(255) | NO   |     | NULL    |                |
| body        | text         | YES  |     | NULL    |                |
| category_id | int(11)      | YES  |     | NULL    |                |
| tags        | varchar(50)  | YES  |     | NULL    |                |
| mark        | tinyint(4)   | NO   |     | 1       |                |
| created     | datetime     | YES  |     | NULL    |                |
| modified    | datetime     | YES  |     | NULL    |                |
+-------------+--------------+------+-----+---------+----------------+

mysql> describe comments;
+----------+--------------+------+-----+---------+----------------+
| Field    | Type         | Null | Key | Default | Extra          |
+----------+--------------+------+-----+---------+----------------+
| id       | int(11)      | NO   | PRI | NULL    | auto_increment |
| post_id  | int(11)      | NO   | MUL | NULL    |                |
| name     | varchar(255) | NO   |     | NULL    |                |
| email    | varchar(255) | NO   |     | NULL    |                |
| body     | varchar(500) | NO   |     | NULL    |                |
| mark     | tinyint(4)   | NO   |     | 1       |                |
| created  | datetime     | YES  |     | NULL    |                |
| modified | datetime     | YES  |     | NULL    |                |
+----------+--------------+------+-----+---------+----------------+

任何答案将不胜感激。提前致谢。

乔治·贝索斯（Giorgos Betsos）

试试这个：

SELECT p.id, p.title, p.mark,
       c.latest_at     
FROM posts p
LEFT JOIN (
   SELECT post_id, MAX(created) AS latest_at
   FROM comments
   WHERE mark = 1
   GROUP BY post_id 
) AS c ON c.post_id = p.id 
WHERE c.latest_at IS NOT NULL
ORDER BY c.latest_at DESC LIMIT 10

或者只是这样：

SELECT p.id, p.title, p.mark,
       c.latest_at     
FROM posts p
INNER JOIN (
   SELECT post_id, MAX(created) AS latest_at
   FROM comments
   WHERE mark = 1
   GROUP BY post_id 
) AS c ON c.post_id = p.id 
ORDER BY c.latest_at DESC LIMIT 10

因为第一个查询c.latest_at IS NOT NULL的WHERE子句的谓词LEFT JOIN变成了INNER JOIN。

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