如何在单个查询中查询3个表？

Question

I'm really sorry for the first post as i didn't explain everything.

Basically i have 3 tables, One for posts, One for Categories, & Another to link categories with posts.

I want in a single MySQL query to select posts that are under a specific category.

posts(id,title,body)
---------------------
125,Some title,Blah blah


categories(id,name)
---------------------
1,politic
2,entertainment

linker(categoryid,postid)
---------------------
2,125

I want in single query to fetch posts in the entertainment category by example, what to do?

Thanks

Answer 1

select 
     p.*
from 
     posts p
     inner join linker l on l.postid = p.id
     inner join categories c on c.categoryid = l.categoryid
where 
     c.name = 'entertainment'

Answer 2

It's the same exact thing, you just have to join 3 tables intead of 2 :

SELECT P.id post_id,
       P.title,
       P.body,
       C.id category_id,
       C.name
FROM posts P
INNER JOIN linker L
    ON P.id = L.postid
INNER JOIN categories C
    ON L.categoryid = C.id
WHERE C.name = 'Category'

Don't be afraid to do your own tests. If you understand how to join two tables, you should understand how to join three, four and more.

Answer 3

The following SQL statement should provide you with a basis for what you are trying to do.

select p.*
from posts p
inner join linker l
on l.postid = p.id
inner join categories c
on l.categoryid = c.id
where c.name = 'entertainment'

Answer 4

If a post belongs to 2 categories, you can still use pinkfloydx33's query with DISTINCT in the select statement:

select 
     DISTINCT p.*
from 
     posts p
     inner join linker l on l.postid = p.id
     inner join categories c on c.categoryid = l.categoryid
where 
     c.name = 'entertainment'

The result set will show only one record.

Answer 5

If you are specifying only one category in the WHERE clause, then the result will be a single row for each post ID.

Either way you can use DISTINCT or GROUP BY when the result could be more than one row per ID, but in that case i prefer the second one (GROUP BY).