MySQL 查询 - 排序和分组的混合

Question

我正在尝试从表格中获取以下信息:

• 按“b”排序(表 A 升序，表 B 降序)
• 所有具有特定值的“u”首先出现(在此示例中 u=3) - 那么顺序并不重要
• N (NULL) 始终位于末尾
• “r”不能出现两次(但所有不同的“r”必须出现)

对于“ORDER BY IFNULL(u, 99999999) != 3 , u , b”，标准 1、2 和 3 有效，但对于最后一个，我不知道。

使用 DISTINCT 我没有达到目标，就像使用 GROUP BY 一样。

有人有想法吗？非常感谢。

start table          target table A 
+----------------+   +----------------+
| id | r | u | b |   | id | r | u | b |
+----------------+   +----------------+
|  1 | 1 | 1 | 9 |   | 18 | 5 | 3 | 1 |
|  2 | 1 | 3 | 4 |   | 2  | 1 | 3 | 4 |
|  3 | 1 | 4 | 6 |   | 15 | 4 | 3 | 7 |
|  4 | 1 | 5 | 5 |   | 11 | 3 | 3 | 8 |
|  5 | 2 | 1 | 2 |   | *  | 2 | * | * |
|  6 | 2 | 2 | 1 |   | 21 | 6 | N | N |
|  7 | 2 | 4 | 5 |   +----------------+
|  8 | 2 | 5 | 7 |
|  9 | 3 | 1 | 4 |
| 10 | 3 | 2 | 3 |
| 11 | 3 | 3 | 8 |
| 12 | 3 | 5 | 2 |   target table B
| 13 | 4 | 1 | 3 |   +----------------+
| 14 | 4 | 2 | 2 |   | id | r | u | b |
| 15 | 4 | 3 | 7 |   +----------------+
| 16 | 4 | 4 | 6 |   | 11 | 3 | 3 | 8 |
| 17 | 5 | 2 | 9 |   | 15 | 4 | 3 | 7 |
| 18 | 5 | 3 | 1 |   | 2  | 1 | 3 | 4 |
| 19 | 5 | 4 | 6 |   | 18 | 5 | 3 | 1 |
| 20 | 5 | 5 | 4 |   | *  | 2 | * | * |
| 21 | 6 | N | N |   | 21 | 6 | N | N |
+----------------+   +----------------+

Answer 1

SELECT  *
FROM    mytable m
WHERE   id =
        (
        SELECT  id
        FROM    mytable mi
        WHERE   mi.r = m.r
        ORDER BY
                u IS NULL, u = 3 DESC, b, id
        LIMIT 1
        )
ORDER BY
        u IS NULL, u = 3 DESC, b

更新:

SELECT  *
FROM    objects o
LEFT JOIN
        ratings r
ON      r.id = COALESCE(
        (
        SELECT  id
        FROM    ratings
        WHERE   object_id = o.id
                AND user_id = :myuser
        ORDER BY
                rating DESC
        LIMIT   1
        ),
        (
        SELECT  id
        FROM    ratings
        WHERE   object_id = o.id
        ORDER BY
                rating DESC
        LIMIT   1
        )
        )