Question

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.

Answer 1

My old answer is not the shortest, here’s the shortest one:

select p.*, '' as x, t.name, t.name like '%SQL%'
from Posts p
join Posts_tags pt on pt.p_id = p.id 
join Tags t on t.id = pt.t_id;

Output:

ID  TITLE   BODY    X       NAME    T.NAME LIKE '%SQL%'
1   One     text1           SQL     1
1   One     text1           PHP     0
2   Two     text2           SQL     1
3   Three   text3           GLSL    0

So if we group by ID, and check that if at least one (aided by bit_or; Postgresql has this too, aptly named bool_or) of the elements in the group satisfied the ‘%SQL%’ criteria, its bit is ON (aka boolean = true). We can pick that group and we retain all the tags under that group, example, tag id 1 appear on post 1, and post 1 has other tags, which is #3 or PHP. All tags that belong to same Post ID will not be discarded, as we will not be using WHERE filter, we will be using HAVING filter instead:

select p.*, group_concat(t.name) as tags
from Posts p
join Posts_tags pt on pt.p_id = p.id 
join Tags t on t.id = pt.t_id
group by p.id
having bit_or(t.name like '%SQL%');

We can also rewrite that to this:

select p.*, group_concat(t.name) as tags
from Posts p
join Posts_tags pt on pt.p_id = p.id 
join Tags t on t.id = pt.t_id
group by p.id
having sum(t.name like '%SQL%') >= 1;

BIT_OR is like IN, or ANY, so it’s more semantic than evaluating things by SUM

Output:

D   TITLE   BODY    TAGS
1   One     text1   PHP,SQL
2   Two     text2   SQL

Live test: http://www.sqlfiddle.com/#!2/52b3b/26

---

I’m learning so much on stackoverflow. After my old answer, I’m thinking how to make an equivalent shorter code in Postgresql using windowing function(which MySQL don’t have) via SUM OVER partition. Then I thought of Postgresql’s bool_or,bool_and and every function. Then I remember MySQL has bit_or 🙂

The last solution using SUM is just an afterthought, when I thought up that bit_or is just a semantic of at least one is true, then it’s obvious that you can use HAVING SUM(condition) >= 1 too. Now it works on all database 🙂

I ended up not solving it by windowing function, the solution above now works on all database 🙂