MySQL左连接WHERE table2.field ="X" [英] MySQL Left join WHERE table2.field = "X"

问题描述

我有下表:

页面:

+------------+--------------+------+-----+---------+----------------+
| Field      | Type         | Null | Key | Default | Extra          |
+------------+--------------+------+-----+---------+----------------+
| page_id    | int(11)      | NO   | PRI | NULL    | auto_increment |
| type       | varchar(20)  | NO   |     | NULL    |                |
| categories | varchar(255) | NO   |     | NULL    |                |
| title      | varchar(255) | NO   | MUL | NULL    |                |
| text       | longtext     | NO   | MUL | NULL    |                |
+------------+--------------+------+-----+---------+----------------+

自定义:

+---------+------------------+------+-----+---------+-------+
| Field   | Type             | Null | Key | Default | Extra |
+---------+------------------+------+-----+---------+-------+
| page_id | int(10) unsigned | NO   | PRI | NULL    |       |
| key     | varchar(255)     | NO   | PRI | NULL    |       |
| value   | longtext         | NO   |     | NULL    |       |
+---------+------------------+------+-----+---------+-------+

我想通过以下方式加入表:
1)返回第一个表中的所有条目LEFT JOIN custom ON pages.page_id = custom.page_id
2)pages.type IN ('type_a', 'type_b', 'type_c')
3)第二个表中的键"具有值投票" custom.key = 'votes'

I want to join the tables in a way where:
1) all the entries from the first table are returned LEFT JOIN custom ON pages.page_id = custom.page_id
2) pages.type IN ('type_a', 'type_b', 'type_c')
3) "key" from the second table has value "votes" custom.key = 'votes'

到目前为止，我已完成了所有工作，但第三个条件是问题.如果表自定义中没有key = 'votes'的条目，则查询仅返回包含条目的条目.如果缺少条目，我想返回NULL.

I made everything so far, but the third condition is the problem. If there isn't entry for key = 'votes' in table custom the query returns only these with entries. I want to return NULL if missing entries.

我需要key = 'votes'，因为我还有此page_id的其他条目，键不是投票"，并且这会复制页面

I need key = 'votes', because I have other entries for this page_id where the key is not 'votes' and this duplicates the rows from pages

推荐答案

只需将约束custom.key='votes'添加到LEFT JOIN

SELECT * 
FROM pages LEFT JOIN custom 
     ON pages.page_id=custom.page_id AND custom.key='votes' 
WHERE pages.type IN('type_a','type_b','type_c') ;

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