How can I query a many-to-many join with filtering on the same relation?

manpreet Best Answer 2 years ago

I simplified a many-to-many relationship case with these mockup tables.

Posts:
------------------------------
|   id |    title |     body |
------------------------------
|    1 |      One |    text1 |
|    2 |      Two |    text2 |
|    3 |    Three |    text3 |
------------------------------

Tags:
-------------------
|   id |     name |
-------------------
|    1 |      SQL |
|    2 |     GLSL |
|    3 |      PHP |
-------------------

Post_tags:
------------------------------
|   id |    p_id |      t_id |
------------------------------
|    1 |       1 |         1 |
|    2 |       1 |         3 |
|    3 |       2 |         1 |
|    3 |       3 |         2 |
------------------------------

My goal is to query POSTS with specific TAGS, which I have no problem with, but I also want to display all related tags to the post not just the one I queried for. My query looks like this:

SELECT p.Title, p.Body, t.name
FROM Posts p
LEFT JOIN Post_tags pt ON p.id = pt.p_id
LEFT JOIN Tags t ON t.id = pt.t_id
WHERE t.name LIKE '%SQL%'

This gets the posts with the "SQL" tag, but it only joins the posts table with tags where it found the "SQL" string, so the other tag "PHP" associated with the post doesn't get joined.

Obviously the problem is I'm joining the table on the WHERE clause, but how would one solve this in one query or (preferably with subqueries)?

Currently I'm doing this in two separate queries in my application, one for selecting matching posts and another one that is retreiving full post data. This isn't so efficient and also seems like a lame solution, and I haven't found a better yet, so I decided to ask the StackOverflow community.