MySQL查询 - 排序和分组

Question

我'想从一个表中的以下信息混合物：

• 由 “B” 排序（表A上升，表B下降）
• 所有“U “具有特定值首先出现（在这个例子中u = 3） - 那么顺序并不重要
• N（NULL）总是在末尾
• 不可能发生两次（但所有不同的”r“必须存在）

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，我没有任何线索。

使用DISTINCT我没有达到目标，就像GROUP BY一样。

有没有人有想法？ 非常感谢提前。

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 
     ) 
     )