MySQL联接：从子表中选择一行满足条件，否则为NULL

Question

I am trying to join two tables but I am having some trouble. This is the main table, packs

And here is the second table, media

The media is for storing images of packs . Each pack can have 0 or more images. When the pack has images, one of them will have the is_default field set to 1 to indicate the main image to show.

I want to get all pack and one image for each. If there are no images, then just a simple NULL , otherwise get the image that is is_default .

Here is my query.

SELECT 
    pack.*,
    ( SELECT media.src FROM packs pack LEFT JOIN packs_media media ON pack.id = media.pack_id WHERE media.is_default = 1 GROUP BY media.id LIMIT 1 ) AS image 
FROM packs pack 
LEFT JOIN packs_media ON media.pack_id = pack.id 
GROUP BY pack.id 
ORDER BY pack.id DESC

I have a total of three packs, and only one of them has some images. The query returns 3 results. The problem is that all three results have the same image , when only one should and the other two should have the image field null/empty.

Is there any way of doing this with one query only ? I want to avoid querying the media table in a loop.

Thank you

Answer 1

You need a correlated subquery rather than a join:

SELECT p.*,
      (SELECT m.src
       FROM packs_media m
       WHERE p.id = media.p 
       ORDER BY m.is_default DESC, m.id
       LIMIT 1
      ) AS image 
FROM packs p 
ORDER BY p.id DESC;

In fact, from the description of the problem, no JOIN is necessary in the outer query either. So the GROUP BY is not needed either.

Note the change to the ORDER BY clause. This guarantees that the default value is chosen first.

And, your query gets the same value for all rows because the subquery is independent of the outer query. So, the same value is always chosen.

Answer 2

I had a similar problem lately, and it has a really-really sweet solution, to be honest. At first glance, an obvious try would be adding the WHERE condition is_default = 1 . However, if we do this, where there are no images, the condition won't be met, meaning where it should return NULL , it just skips that row.

However, if you add this condition to the LEFT JOIN part, as I did, it works like a charm. It's beacause if the LEFT JOIN conditions met, it returns the image, and if it doesn't, it returns NULL , per the definition of LEFT JOIN .

So the result should be:

SELECT pack.*, media.src
FROM packs pack 
LEFT JOIN media
ON media.pack_id = pack.id
AND media.is_default = 1

And I think you shouldn't add GROUP BY it just messes things up.

If you want to solve your problem even when there's no is_default ... well, that is a harder nut to crack, but I think I have a solution which won't dwell into long lines of conditions and subqueries. Sadly, I have the feeling that this problem can't be solved without a subquery. My idea was not to choose the item whose media.is_default equals to 1 , rather which has the highest media.is_default . As follows:

SELECT pack.*, media.src
FROM packs pack 
LEFT JOIN media
ON media.pack_id = pack.id
AND media.id = 
   (SELECT m.id FROM media m
    WHERE m.pack_id = media.pack_id
    ORDER BY m.is_default DESC
    LIMIT 1
   )

EXPLANATION: This is how it works: we order all by is_default in a descending order, and refer back to the outer query to use only the current pack_id (I hope it works this way.) If there is a 1, it obviously will be the first item, otherwise it will be a random item, most likely the first in order of ID. And you just take that element and use it. I cannot guarantee if it works at all. Try using IN instead of = as a possible bug fix.

However, I would NOT advise this solution. I'd rather change the previous logic to avoid this problem (eg when the user adds a media to an empty pack, is_default = 1 on that picture).