Question

我有两个sql表： wall 表和 tag 表。他们每个人都与具有“ _和_belongs_to_many”关系相关联。标签表也具有唯一的名称。

这是sql中的表

mysql> describe tags;
+------------+--------------+------+-----+---------+----------------+
| Field      | Type         | Null | Key | Default | Extra          |
+------------+--------------+------+-----+---------+----------------+
| id         | bigint(20)   | NO   | PRI | NULL    | auto_increment |
| name       | varchar(255) | NO   | UNI | NULL    |                |
| count      | int(11)      | NO   |     | NULL    |                |
| created_at | datetime     | NO   |     | NULL    |                |
| updated_at | datetime     | NO   |     | NULL    |                |
+------------+--------------+------+-----+---------+----------------+

mysql> describe tags_walls;
+---------+------------+------+-----+---------+-------+
| Field   | Type       | Null | Key | Default | Extra |
+---------+------------+------+-----+---------+-------+
| tag_id  | bigint(20) | NO   | MUL | NULL    |       |
| wall_id | bigint(20) | NO   |     | NULL    |       |
+---------+------------+------+-----+---------+-------+

mysql> describe walls;
+------------+--------------+------+-----+---------+----------------+
| Field      | Type         | Null | Key | Default | Extra          |
+------------+--------------+------+-----+---------+----------------+
| id         | bigint(20)   | NO   | PRI | NULL    | auto_increment |
| name       | varchar(255) | NO   |     | NULL    |                |
| created_at | datetime     | NO   |     | NULL    |                |
| updated_at | datetime     | NO   |     | NULL    |                |
+------------+--------------+------+-----+---------+----------------+

我正在使用Rails 5，我想查询具有多个标签的墙。

我正在尝试

result = Wall.all.includes(:tags).where(tags: {name: 'TAG1'})
result = result.where(tags: {name: 'TAG2'})

，由rails构造的查询是

SELECT  DISTINCT `walls`.`id`
FROM `walls`
LEFT OUTER JOIN `tags_walls` ON `tags_walls`.`wall_id` = `walls`.`id`
LEFT OUTER JOIN `tags` ON `tags`.`id` = `tags_walls`.`tag_id`
WHERE `tags`.`name` = 'TAG1' AND `tags`.`name` = 'TAG2'

它应该给我多堵墙，但返回值为#<ActiveRecord::Relation []>

我想构建一个自定义的sql查询，然后执行

Wall.includes(:tags).where query

如何对由AND链接的多个条件的联接表进行WHERE查询？

Answer 1

我会这样写：

SELECT tw.id
FROM tags_walls tw JOIN
     tags t
     ON t.id = tw.tag_id
WHERE t.name IN ('TAG1', 'TAG2')
GROUP BY tw.id
HAVING COUNT(*) = 2;

这假定标签在墙上没有重复。如果可能的话，请使用COUNT(DISTINCT t.name) = 2。

注意：

walls不需要，因此JOIN被删除。
您正在寻找匹配项，因此INNER JOIN比LEFT JOIN更合适。
表别名使查询更易于编写和阅读。
不必要的反引号使查询更难编写和阅读。

Answer 2

WHERE tags.name = 'TAG1' OR tags.name = 'TAG2'

或

WHERE tags.name IN ('TAG1','TAG2')

Answer 3

SELECT w.id
  FROM walls w
  JOIN tags_walls tw
    ON tw.wall_id = w.id
  JOIN tags t
    ON t.id = tw.tag_id
   AND t.name IN('TAG1','TAG2')
 GROUP 
    BY w.id 
HAVING COUNT(DISTINCT t.name) = 2 -- where '2' equals the number of arguments in IN()

对具有多个条件的JOINED表的SQL查询

3 个答案: