从多对多关系的末尾选择数据

Question

I have a database of posts, each of which have tags. These tables are named Posts and Tags respectively. I also have a third table, called Posts_Tags which maintains a many-to-many relationship between these two tables.

In order to do this, both my posts and my tags tables have an id column. My Posts_Tags table, therefore, has both a postid and tagid column to store the mappings.

I am querying, for example, all posts with the word "class" in the title. I can do this easily with this query:

SELECT * FROM Posts WHERE title LIKE '%{class}%'

However, now I want to query all posts which not only have "class" in the title, but are also tagged with the "Java" tag. I could do this in two separate queries, where I first get the id of the Java tag:

SELECT id FROM Tags WHERE name='Java'

Then I could plug that into my first query, like this:

SELECT     Posts.* 
FROM       Posts 
INNER JOIN Posts_Tags 
        ON Posts.id=Posts_Tags.postid 
WHERE      Posts_Tags.tagid='$java_tag_id' 
           AND title LIKE '%{class}%'

However, I know I can do this in a single query, I just don't know how. I still have to think a lot about joins when doing just one, and doing multiple joins in the same query makes my head spin. How should I structure my query to perform this operation?

Answer 1

    SELECT p.* 
      FROM Posts p 
      JOIN Posts_Tags pt
        ON pt.postid = p.id
      JOIN tags t
        ON t.id = pt.tagid
     WHERE t.tag='java'
       AND p.title LIKE '%{class}%';

Answer 2

SELECT 
    p.*
FROM posts as p 
INNER JOIN Posts_Tags pt ON pt.post_id = p.id
INNER JOIN  Tags as t ON pt.tags_id = t.id
WHERE   t.tag='java'
AND p.title LIKE '%keyword%';