如何在我的查询中加入映射表

Question

我的查询试图解决表格中的所有问题。

问题 帖子 话题 主题映射

我的标签表设置有第3个表，用于将问题ID与主题ID映射。

但是，如何使用JOIN语句

提取主题表中存储的主题名称

所以基本上我不知道如何在只有主题ID而不是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