UNION on 2 tables returns one almost empty result every time

Question

I have something called Galleries that group media together. That media can be photos or videos. I store photos in one table and videos in another, so I am using a UNION query to find photos and videos that belong to a gallery.

My problem seems to be that my result contains an empty object (no ID) for one of the tables — to rephrase, it will always return a useless result for one of the tables being queried if there are no results in that table.

First, the query:

SELECT * from (
    SELECT g.id AS gallery_id,  'photo' AS type, p.id AS id, p.filename, p.caption, null AS title, null AS service, null AS embed, null AS width, null AS height, p.display_order FROM galleries g 
        LEFT OUTER JOIN photos AS p ON p.gallery_id = g.id
        WHERE g.id = {$this->id}
    UNION
    SELECT g.id AS gallery_id, 'video' AS type, v.id AS id, null AS filename, null AS caption, v.title, v.service, v.embed, v.width, v.height, v.display_order FROM galleries g 
        LEFT OUTER JOIN videos AS v ON v.gallery_id = g.id 
        WHERE g.id = {$this->id}
) AS u ORDER BY display_order;

I am adding the type column so that I may identify what kind of result I get back. I have nulled the results that are not common between the tables.

Like I said, it works, but not quite as expected. If I have a gallery that contains only photos, I still get an (almost) empty video result. An example result:

[] => Galleries Object
    (
        [id] => 
        [name] => 
        [slug] => 
        [gallery_id] => 32
        [type] => video
        [filename] => 
        [caption] => 
        [title] => 
        [service] => 
        [embed] => 
        [width] => 
        [height] => 
        [display_order] => 
    )

[39] => Galleries Object
    (
        [id] => 39
        [name] => 
        [slug] => 
        [gallery_id] => 32
        [type] => photo
        [filename] => 39-studio-blue-pacific.jpg
        [caption] => 
        [title] => 
        [service] => 
        [embed] => 
        [width] => 
        [height] => 
        [display_order] => 1
    )

The first result, labelled with [type]=>video is empty, I call it, because it does not have an ID of a video, title, embed code, etc... It only contains the gallery_id and type .

This was the most complicated query I have put together so far, and I am sure there is something that I am missing. If a gallery only contains videos or only contains photos, I'd like the results to reflect that.

To be a hack, I could check to see if there is an ID when I foreach on these results before I echo something, but I know that my query could be improved instead. Help?

Answer 1

The reason you are getting these results back is because you are using an outer join on the table. Try this instead:

SELECT * from (
    SELECT g.id AS gallery_id,  'photo' AS type, p.id AS id, p.filename, p.caption, null AS title, null AS service, null AS embed, null AS width, null AS height, p.display_order FROM galleries g 
        JOIN photos AS p ON p.gallery_id = g.id
        WHERE g.id = {$this->id}
    UNION
    SELECT g.id AS gallery_id, 'video' AS type, v.id AS id, null AS filename, null AS caption, v.title, v.service, v.embed, v.width, v.height, v.display_order FROM galleries g 
        JOIN videos AS v ON v.gallery_id = g.id 
        WHERE g.id = {$this->id}
) AS u ORDER BY display_order;

Also, it sounds like you might get a lot of benefit from reading this article I wrote about SQL to help you understand the concepts better.