MYSQL - 在多个条件下左加入 - 媒体、标签和标签链接

Question

I couldn't find anything that matches my issues.

I'm trying to improve my files search query to be able to fetch images by multiple tags (AND and OR).

Fetching images that have one or the other tag is rather easy:

SELECT M.*, MTL.tag_id 
FROM medias M 
LEFT JOIN medias_taglinks MTL ON MTL.media_id=M.id 
WHERE ( MTL.tag_id = '1' || MTL.tag_id = '2') 
GROUP BY M.id

But if I want to fetch a image that has both of those tags, I get no results and I'm failing to understand why.

SELECT M.*, MTL.tag_id 
FROM medias M 
LEFT JOIN medias_taglinks MTL ON MTL.media_id=M.id 
WHERE ( MTL.tag_id = '1' && MTL.tag_id = '2') 
GROUP BY M.id

I'm guessing I need to add my condition to the join but I haven't succeeded yet.

SQL FIDDLE HERE: http://sqlfiddle.com/#!9/6e5d6/4

Answer 1

Checking for an image to have two different tags requires comparing values across rows, which, in turns, suggest aggregation:

select m.id, m.name
from medias m
inner join medias_taglinks t on t.media_id = m.id
where t.tag_id in (1, 2)
group by m.id, m.name
having min(t.tag_id) <> max(t.tag_id)

The query filters on the two searched tags, aggregates by media, and ensures that both tags are present.

In your db fiddle, this produces:

id | name
-- | ----
1  | Media 1

If you want to handle more than two tags, here is a more generic approach:

select m.id, m.name
from medias m
inner join medias_taglinks t on t.media_id = m.id
where t.tag_id in (1, 2, 3, 4)
group by m.id, m.name
having count(*) = 4

This assumes no duplicate (media_id, tag_id) in the tag table; otherwise, you need having count(distinct t.tag_id) = 4 .

Side note: the left join seems unnecessary; if you want to search medias for tags, you need to bring the tab table anyway.