MySQL - LEFT JOIN value of columnX where MAX(columnY) - mysql

Question

So this is my query:
SELECT SQL_CALC_FOUND_ROWS wagons.id,
wagons.mid,
wagons.year,
wagons.make,
wagons.model,
wagons.nickname,
wagons.description,
members.first,
members.last,
wagon_photos.filename,
Count(DISTINCT( likes.id )) AS likes,
Count(DISTINCT( comments.id )) AS comments
FROM wagons
INNER JOIN members
ON members.mid = wagons.mid
LEFT JOIN wagon_photos
ON wagon_photos.wid = wagons.id
LEFT JOIN likes
ON likes.wid = wagons.id
LEFT JOIN comments
ON comments.wid = wagons.id
GROUP BY wagons.id
ORDER BY wagons.id DESC
LIMIT 10
I am trying to fetch the wagon_photos.filename where wagon_photos.default is the maximum value. (It's a boolean, where only one row (for each wagon_photos.wid) will be 1.) On the off-chance the member does not have a default photo selected, I'd like it to return the lowest wagon_photos.id if possible.
I have tried numerous queries with ORDER BY and GROUP BY, but I think it may be a little more complicated than that. If I use a WHERE clause, it left out any wagons that did not have any photos linked to it, which I do not want to do.
CREATE TABLE `wagons` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`mid` int(8) DEFAULT NULL,
`year` varchar(4) DEFAULT NULL,
`make` varchar(50) DEFAULT NULL,
`model` varchar(100) DEFAULT NULL,
`nickname` varchar(200) DEFAULT NULL,
`description` mediumtext,
`featured` int(1) DEFAULT '0',
`visibility` int(1) DEFAULT '1',
`commision` int(1) DEFAULT '1',
`feat1title` varchar(50) DEFAULT NULL,
`feat2title` varchar(50) DEFAULT NULL,
`feat3title` varchar(50) DEFAULT NULL,
`feat4title` varchar(50) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=617 DEFAULT CHARSET=utf8;
CREATE TABLE `wagon_photos` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`wid` int(11) DEFAULT NULL,
`filename` varchar(255) DEFAULT NULL,
`caption` varchar(255) DEFAULT '',
`default` int(1) DEFAULT '0',
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=41 DEFAULT CHARSET=utf8;

Answer 1

Let me try and understand what you're asking. I think what you're asking is "how do I construct a query where I want to select all of the items that have the max value of a particular column". If that's the case, you want to break this down into simpler units in order to solve it. First, you'll need the max value itself:
select max(wp.default) as max_val from wagon_photos wp
This gets you the max value.
Next you can create a query to select rows where a condition is true, but leave the condition out for a moment:
select (your rows) from wagons w where (a condition will go here)
And your condition is that you want wagon_photos.default to equal the max value. So you'll need to join in that table:
select (your rows) from wagons w join wagon_photos wp
on wp.wid = w.id where wp.default = (something)
Your "something" happens to be that max_value that we created a query for in the beginning. So putting this together, you'll use a subquery which will look like this:
select (your rows) from wagons w join wagon_photos wp
on wp.wid = w.id where wp.default = (
select max(wpb.default) from wagon_photos wpb
)
I haven't set up the tables myself to run this, so I might have goofed on parentheses, and it doesn't include all your joins. But I think this should point you in the right direction.

Answer 2

Well I feel a little bit silly now.
I finally thought of adding AND wagon_photos.default = 1 after the LEFT JOIN for wagon_photos and it works. I just need to make sure they always have a default setup, so I ran a script that on every update to their photos, it checks for a default, and if one isn't set, set the newest upload as default.
... LEFT JOIN wagon_photos ON wagon_photos.wid = wagons.id AND wagon_photos.default = 1 ...