通过 JOIN 和 GROUP BY 获取最近的行？

Question

这是表album，它的结构和数据是，

这是另一个表photos，它的结构和数据是，

我想列出所有相册名称、相应相册的总照片、按 JOIN 和 GROUP BY 上传的最后一张图片。

这是我现在使用的 Codeigniter 查询。一切正常，除了它获取image 列值的第一行而不是最新行。

 $query = $this->db->select('albums.name, photos.image, albums.created, albums.id, 
    COUNT(photos.id) as total')
   ->from('photos')
   ->join('albums', 'albums.id = photos.album_id')  
   ->order_by('photos.id', 'desc')
   ->group_by('photos.album_id')                       
   ->limit($limit, $offset)
   ->get();

上面生成的 SQL 查询是，

SELECT `albums`.`name`, `photos`.`image`, `albums`.`created`, `albums`.`id`, COUNT(photos.id) as total FROM `photos` JOIN `albums` ON `albums`.`id` = `photos`.`album_id` GROUP BY `photos`.`album_id` ORDER BY `photos`.`id` DESC LIMIT 10

对于这两个查询，结果都是，

[{"名称":"电影 Stills","image":"assets/images/albums/2016/01/84786b91857b45fa9391943e9a468ac6.jpg","created":"2016-01-19 23:41:53","id":"3","total":"3"},{"name":"Firstlook","image":"assets/images/albums/2016/01/6e90cdfdec444dfc53f4a7c30a2ac31e。 jpg","创建":"2016-01-19 23:16:05","id":"1","total":"2"}]

如果您查看图像键，它会显示第一个而不是最后一行的值。

我对这两种查询解决方案（Codeigniter/SQL）都持开放态度。

Answer 1

$query = $this->db->select('albums.name, ph.image, albums.created, albums.id, 
    COUNT(ph.id) as total')
                ->from('photos as ph') // added prefix
                ->join('albums', 'albums.id = ph.album_id')
                ->where('ph.id = (SELECT MAX(ph1.id) FROM photos as ph1 WHERE ph1.album_id=ph.album_id)',NULL,FALSE) // subquery to get latest record
                ->order_by('ph.id', 'desc')
                ->group_by('ph.album_id')
                ->limit($limit, $offset)
                ->get();

where() 中的 NULL,FALSE 告诉 CodeIgniter 不要逃避查询，这可能会搞砸 Sub query in CodeIgniter

或者，SQL 查询是

SELECT `albums`.`name`, `ph`.`image`, `albums`.`created`, `albums`.`id`, COUNT(ph.id) as total FROM `photos` as `ph` JOIN `albums` ON `albums`.`id` = `ph`.`album_id` WHERE ph.id IN (SELECT MAX(ph1.id) FROM photos as ph1 WHERE ph1.album_id=ph.album_id) GROUP BY `ph`.`album_id` ORDER BY `ph`.`id` DESC LIMIT 10

Answer 2

您按photos.album_id 分组，但创建的日期字段不受 agrregate 函数的约束，因此 mysql 根据文档获取随机行的值（好吧，它不是那么随机，它拾取它在扫描数据时遇到的第一个对应值）

在创建的日期字段上使用 max() 来检索最新日期：

    SELECT max(`albums`.`name`) as name, `photos`.`image`, max(`albums`.`created`) as latest_date, `albums`.`id`, COUNT(photos.id) as total
FROM `photos` JOIN `albums` ON `albums`.`id` = `photos`.`album_id`
GROUP BY `photos`.`album_id`
ORDER BY `photos`.`id` DESC LIMIT 10

photos.image 也会遇到同样的问题。要获取与最新创建日期相关联的图像，您需要一个子查询。

select a.name, p.image, t.mdate
from albums a
inner join (select album_id, max(created_date) as mdate from photos group by album_id) t on a.id=t.album_id
inner join photos p on a.id=p.album_id and t.mdate=p.created_date

Answer 3

经过多次尝试，我找到了解决问题的方法。这是一个简单的查询，

$query = $this->db->query("SELECT p.`image`, COUNT(p.`id`) as total, a.`id`, a.`created`, a.`name` FROM `albums` as a JOIN (SELECT * FROM `photos` as p1 ORDER BY id DESC) p ON a.`id` = p.`album_id` GROUP BY p.`album_id` ORDER BY p.`id` DESC LIMIT 10");

我使用了@Shadow 所说的子查询，我只是对子查询的列Id 进行了 ORDER BY。通过这个，一切都按预期工作。