Mysql 如何指定分组依据应保留哪些行以及应折叠哪些行

我正在使用的应用程序包含一部分，用户可以在其中创建相册并将图像上载到相册。应用程序会创建图像的多个大小调整，以便不向用户提供大版本。有关文件的所有信息都以以下结构存储在数据库中

相册表具有id和名称

照片表有一个照片列表，每个照片都知道它属于哪个相册

还有一张照片。每个photo_版本存储photo表中对象的ID

此逻辑由以下模式表示：

CREATE TABLE albums(`id` int, `name` varchar(255)); 
INSERT INTO albums (id, name) VALUES 
(1, "one"),
(2, "two"),
(3, "three");

CREATE TABLE photos(`id` int, `albums_id` int, `title` varchar(255));
INSERT INTO photos (id, albums_id, title) VALUES 
(1, 1, "a"),
(2, 1, "b"),
(3, 1, "c");

CREATE TABLE photos_versions(`id` int, `photos_id` int, `width` int, `height` int);
INSERT INTO photos_versions (photos_id, width, height) VALUES 
(1, 1000, 800),(1, 800, 600), (1, 600, 400),
(2, 1000, 800), (2, 800, 600), (2, 600, 400),
(3, 1000, 800), (3, 800, 600), (3, 600, 400);

用户界面能够请求特定的高度，我正在处理的后端应该返回数据库中最接近的现有高度。我正在按要求做这件事。它从连接所有这些表开始：

SELECT *
FROM albums a
INNER JOIN photos p ON p.albums_id = a.id
INNER JOIN photos_versions pv ON pv.photos_id = p.id;

结果如下表所示：

+------+------+----+-----------+-------+------+-----------+-------+--------+
| id   | name | id | albums_id | title | id   | photos_id | width | height |
+------+------+----+-----------+-------+------+-----------+-------+--------+
|    1 | one  |  1 |         1 | a     | NULL |         1 |  1000 |    800 |
|    1 | one  |  1 |         1 | a     | NULL |         1 |   800 |    600 |
|    1 | one  |  1 |         1 | a     | NULL |         1 |   600 |    400 |
|    1 | one  |  2 |         1 | b     | NULL |         2 |  1000 |    800 |
|    1 | one  |  2 |         1 | b     | NULL |         2 |   800 |    600 |
|    1 | one  |  2 |         1 | b     | NULL |         2 |   600 |    400 |
|    1 | one  |  3 |         1 | c     | NULL |         3 |  1000 |    800 |
|    1 | one  |  3 |         1 | c     | NULL |         3 |   800 |    600 |
|    1 | one  |  3 |         1 | c     | NULL |         3 |   600 |    400 |
+------+------+----+-----------+-------+------+-----------+-------+--------+
9 rows in set (0.00 sec)

+------+------+----+-----------+-------+------+-----------+-------+--------+
| id   | name | id | albums_id | title | id   | photos_id | width | height |
+------+------+----+-----------+-------+------+-----------+-------+--------+
|    1 | one  |  1 |         1 | a     | NULL |         1 |  1000 |    800 |
|    1 | one  |  2 |         1 | b     | NULL |         2 |  1000 |    800 |
|    1 | one  |  3 |         1 | c     | NULL |         3 |  1000 |    800 |
+------+------+------+-----------+-------+------+-----------+-------+--------+
3 rows in set (0.00 sec)

现在，我们需要按照片的id进行分组（因为我们希望得到最接近给定照片的版本）。因此，请求变成：

SELECT *
FROM albums a
INNER JOIN photos p ON p.albums_id = a.id
INNER JOIN photos_versions pv ON pv.photos_id = p.id
GROUP BY photos_id;

其结果如下表所示：

+------+------+----+-----------+-------+------+-----------+-------+--------+
| id   | name | id | albums_id | title | id   | photos_id | width | height |
+------+------+----+-----------+-------+------+-----------+-------+--------+
|    1 | one  |  1 |         1 | a     | NULL |         1 |  1000 |    800 |
|    1 | one  |  1 |         1 | a     | NULL |         1 |   800 |    600 |
|    1 | one  |  1 |         1 | a     | NULL |         1 |   600 |    400 |
|    1 | one  |  2 |         1 | b     | NULL |         2 |  1000 |    800 |
|    1 | one  |  2 |         1 | b     | NULL |         2 |   800 |    600 |
|    1 | one  |  2 |         1 | b     | NULL |         2 |   600 |    400 |
|    1 | one  |  3 |         1 | c     | NULL |         3 |  1000 |    800 |
|    1 | one  |  3 |         1 | c     | NULL |         3 |   800 |    600 |
|    1 | one  |  3 |         1 | c     | NULL |         3 |   600 |    400 |
+------+------+----+-----------+-------+------+-----------+-------+--------+
9 rows in set (0.00 sec)

+------+------+----+-----------+-------+------+-----------+-------+--------+
| id   | name | id | albums_id | title | id   | photos_id | width | height |
+------+------+----+-----------+-------+------+-----------+-------+--------+
|    1 | one  |  1 |         1 | a     | NULL |         1 |  1000 |    800 |
|    1 | one  |  2 |         1 | b     | NULL |         2 |  1000 |    800 |
|    1 | one  |  3 |         1 | c     | NULL |         3 |  1000 |    800 |
+------+------+------+-----------+-------+------+-----------+-------+--------+
3 rows in set (0.00 sec)

但是，它不一定将行与属性保持在一起（高度最接近我指定的高度）。我如何根据照片编号进行分组并选择最接近高度的照片

---

附加了SQL FIDLE-

这里的一种方法是在查询中添加一个附加连接，该连接将限制每个照片id组具有最接近高度的照片

SELECT a.*, p.*, pv1.*
FROM albums a
INNER JOIN photos p
    ON p.albums_id = a.id
INNER JOIN photos_versions pv1
    ON pv1.photos_id = p.id
INNER JOIN
(
    SELECT photos_id, MIN(ABS(height - SOME_HEIGHT)) AS diff
    FROM photos_versions
    GROUP BY photos_id
) pv2
    ON pv1.photos_id = pv2.photos_id AND
       MIN(ABS(pv1.height - SOME_HEIGHT)) = pv2.diff

---

您可以将

某些\u HEIGHT

替换为通过高度搜索获得的任何值。

什么决定了应该返回哪一行？您在查询中指定的高度在哪里？@sgedes我没有，这就是我的问题所在