帖子和标签-加入限制

Question

I have 3 tables: posts , tags , *posts_tags* . I want to list posts, and all tags associated with them, but to limit the results.

This is what I do now:

SELECT 
    p.*,
    t.*
FROM 
    (
        SELECT * FROM  posts LIMIT 0, 10
    ) as p
    LEFT JOIN
        posts_tags as pt
        ON pt.post_id = p.post_id
    LEFT JOIN
        tags as t
        ON t.tag_id = pt.tag_id

It is working fine, but it seems to be a little bit slow..

Is there a better/faster way of doing this? Can I apply LIMIT somewhere else for better results?

EDIT: I want to limit posts, and not results. A post can have many tags.

Answer 1

SELECT 
    p.*,
    t.*
FROM 
    posts as p
    LEFT JOIN
        posts_tags as pt
        ON pt.post_id = p.post_id
    LEFT JOIN
        tags as t
        ON t.tag_id = pt.tag_id
LIMIT 0, 10

Should work ;)

EDIT

MySQL is quite slow when running multiple joins, in my opinion it's better to separate your query into two and then join the result in your app code (application overhead should not be so big since its only 10 results).

Answer 2

Try running your query with the EXPLAIN keyword in front of it:

EXPLAIN SELECT ...

This will give you and idea about how MySQL is executing your query. Maybe you miss a key or an index somewhere. Here's how to read the result of EXPLAIN: http://dev.mysql.com/doc/refman/5.5/en/explain-output.html

Answer 3

Have you tried moving the limiting subquery to the where clause instead:

SELECT 
    p.*,
    t.*
FROM 
    posts as p
LEFT JOIN
    posts_tags as pt
    ON pt.post_id = p.post_id
LEFT JOIN
    tags as t
    ON t.tag_id = pt.tag_id
WHERE
    p.post_id in (select post_id from post limit 0,10)