MySQL连接：即使secon表没有匹配数据，如何从一个表中获取结果

Question

So, I have two tables with 'articles' and 'tags' I am trying to fetch articles and their corresponding tags, with this code:

       SELECT b.id, b.title, b.content, b.slug, b.created_at, GROUP_CONCAT(t.name) as tags
FROM blogs b INNER JOIN 
     tags t
     ON b.id = t.blog_id
GROUP BY b.id, b.title;

The problem is that if an blogs has no tag, then I get no blogs, instead of getting the blog with no tags, how is this achievable

Answer 1

You can use left join :

SELECT b.id, b.title, b.content, b.slug, b.created_at, 
       GROUP_CONCAT(t.name) as tags
FROM blogs b LEFT JOIN 
     tags t
     ON b.id = t.blog_id
GROUP BY b.id;

Normally, selecting a bunch of columns with a GROUP BY would be bad style. However, blogs.id is (presumably) unique in blogs , so this is the one case where selecting other columns from blogs is fine.

Answer 2

I think what you are looking for is Left outer Join .

select  b.id,b.title,b.content,b.slug,b.created_at,GROUP_CONCAT(t.name) as tags 
from
blogs b left join tags t 
on b.id = t.blog_id 
group by b.id,b.title;