mysql - 如何在我的查询中加入映射表

Question

我有一个查询试图找出我表中的所有问题。

问题帖子主题主题映射

我的标签表设置了第三个表，用于将问题 ID 与主题 ID 进行映射。

但是，如何使用 JOIN 语句提取存储在主题表中的主题名称

所以基本上我不知道如何在只有主题 ID 而没有主题名称的表上执行 JOIN 语句

SELECT questions.* 
     , posts.post
     , COUNT(posts.post) as total_answers
     , posts.votes
     , posts.id as post_id
     , posts.created
     , users.id as user_id
     , users.username, users.rep
     , topics.name
FROM questions
LEFT JOIN posts ON questions.id = posts.question_id
LEFT JOIN users ON questions.user_id = users.id
LEFT JOIN topics ON topic_mapping.question_id = questions.id
GROUP BY questions.id

非常感谢

Answer 1

您需要先将问题加入映射表。

SELECT questions.* 
     , posts.post
     , COUNT(posts.post) as total_answers
     , posts.votes
     , posts.id as post_id
     , posts.created
     , users.id as user_id
     , users.username, users.rep
     , topics.name
FROM questions
LEFT JOIN posts ON questions.id = posts.question_id
LEFT JOIN users ON questions.user_id = users.id
LEFT JOIN topic_mapping ON questions.id = topic_mapping.question_id
LEFT JOIN topics ON topic_mapping.topic_id = topics.id
GROUP BY questions.id