Question

我有一个表，每次创建一个帖子时都会创建一个记录，另一个表会在每次收到帖子时创建一个记录。

CREATE TABLE `posts` (
  `post_id` bigint(20) NOT NULL AUTO_INCREMENT,
  `title` varchar(100) NOT NULL,
  PRIMARY KEY (`post_id`)
)

CREATE TABLE `favorited_posts` (
  `user_id` bigint(20) NOT NULL,
  `post_id` bigint(20) NOT NULL,
  KEY `user_id` (`user_id`),
  KEY `post_id` (`post_id`),
  FOREIGN KEY (`post_id`) REFERENCES `posts` (`post_id`)
)

以下查询返回一个结果集，该结果集显示帖子被收藏的次数

SELECT p.post_id, p.title,
    COUNT(fp.post_id) as num_favorites
FROM posts p
LEFT JOIN favorited_posts fp
ON fp.post_id = p.post_id
WHERE p.post_id BETWEEN 0 AND 10000
GROUP BY 1;

+-------------------+----------------+
| post_id  | title  |  num_favorites |
+-------------------+----------------+
| 1        | abc    |  15            |
| 2        | hello  |  0             |
| 3        | test   |  7             |
+----------+--------+----------------+

如何更改查询以创建具有true / false，0/1值的ad-hoc列，该值表示用户是否已收藏特定帖子？它会产生这样的结果集（用户已经收到了第2和第3个帖子）

+-------------------+----------------+---------------+
| post_id  | title  |  num_favorites | has_favorited |
+-------------------+----------------+---------------+
| 1        | abc    |  15            | 0             |
| 2        | hello  |  0             | 1             |
| 3        | test   |  7             | 1             |
+----------+--------+----------------+---------------+

我可以检索特定用户在查询中收到的帖子，但不知道如何在捕获post_id，标题和收藏总数的单个查询中检索此信息。

SELECT * FROM favorited_posts
WHERE user_id = 10;

Answer 1

只需使用左连接中的第二个查询：

SELECT p.post_id, p.title, IF(COUNT(fp.post_id)>0, 'True', 'False') as user_favorite
FROM posts p
LEFT JOIN (
        SELECT post_id FROM favorited_posts
        WHERE user_id = 10
    ) fp ON fp.post_id = p.post_id
WHERE p.post_id BETWEEN 0 AND 10000
GROUP BY 1;

另外，您错过了favorited_posts表中user的外键。

Answer 2

您可以使用条件聚合：

SELECT p.post_id,
       p.title,
       COUNT(fp.post_id) as num_favorites,
       sum(case when fp.user_id = 10 then 1 else 0 end) as has_favorited
  FROM posts p
  LEFT JOIN favorited_posts fp
    ON fp.post_id = p.post_id
 WHERE p.post_id BETWEEN 0 AND 10000
 GROUP BY p.post_id, p.title

此查询与您现在的查询相同，不同之处在于它显示的列指示给定用户（在case语句中指定）是否已在给定行上的特定帖子中占用。

如何将表连接到自身，对正在加入的字段进行分组？

2 个答案: