Question

有两个SQL表：

Parents:
+--+---------+
|id|   text  |
+--+---------+
| 1|  Blah   |
| 2|  Blah2  |
| 3|  Blah3  |
+--+---------+

Childs
+--+------+-------+
|id|parent|feature|
+--+------+-------+
| 1|   1  |  123  |
| 2|   1  |   35  |
| 3|   2  |   15  |
+--+------+-------+

我想从Parents表中的每一行中选择单个查询，并从Childs表中选择每一行与“parent” - “id”值和最大“feature”列值的单行。在此示例中，结果应为：

+----+------+----+--------+---------+
|p.id|p.text|c.id|c.parent|c.feature|
+----+------+----+--------+---------+
|  1 | Blah |  1 |    1   |    123  |
|  2 | Blah2|  3 |    2   |    15   |
|  3 | Blah3|null|   null |   null  |
+----+------+----+--------+---------+

其中p =父表和c =子表

我尝试LEFT OUTER JOIN和GROUP BY，但是MSSQL Express告诉我，GROUP BY的查询需要在每个非Groupped字段上使用Aggregate函数。而且我不想将它们全部分组，而是选择顶行（使用自定义排序）。

我完全没有想法......

Answer 1

select p.id, p.text, c.id, c.parent, c.feature
from Parents p
left join (select c1.id, c1.parent, c1.feature
             from Childs c1
             join (select p1.id, max(c2.feature) maxFeature
                     from Parents p1
                left join Childs c2 on p1.id = c2.parent
            group by p1.id) cf on c1.parent = cf.id 
                              and c1.feature = cf.maxFeature) c
on p.id = c.parent

Answer 2

使用CTE（SQL Server 2005 +）：

WITH max_feature AS (
   SELECT c.id,
          c.parent,
          MAX(c.feature) 'feature'
     FROM CHILD c
 GROUP BY c.id, c.parent)
   SELECT p.id,
          p.text,
          mf.id,
          mf.parent,
          mf.feature
     FROM PARENT p
LEFT JOIN max_feature mf ON mf.parent = p.id

非CTE当量：

   SELECT p.id,
          p.text,
          mf.id,
          mf.parent,
          mf.feature
     FROM PARENT p
LEFT JOIN (SELECT c.id,
                  c.parent,
                  MAX(c.feature) 'feature'
             FROM CHILD c
         GROUP BY c.id, c.parent) mf ON mf.parent = p.id

您的问题缺乏处理断路器的详细信息（当2 + CHILD.id值具有相同的特征值时）。 Agent_9191的答案使用了TOP 1，但这将是第一个返回的＆amp;不一定是你想要的那个。

Answer 3

这应该有效：

SELECT p.id, p.text, c.id, c.parent,c.feature
FROM parent p
 LEFT OUTER JOIN (SELECT TOP 1 child.id,
                               child.parent,
                               MAX(child.feature)
                  FROM child
                  WHERE child.parent = p.id
                  GROUP BY child.id, child.parent
                  ) c ON p.id = c.parent

Answer 4

manji的查询不处理最大功能的断路器。这是我的方法，我已经测试过了：

;WITH WithClause AS (SELECT p.id, p.text, 
        (SELECT TOP 1 c.id from childs c 
            where c.parent = p.id order by c.feature desc) 
        AS BestChildID
    FROM Parents p) 
SELECT WithClause.id, WithClause.text, c.id, c.parent, c.feature
FROM WithClause 
LEFT JOIN childs c on WithClause.BestChildID = c.id

Answer 5

如果需要通过关闭嵌套选择来加入与MAX列不同的组以及组中描述的任何列，则可以使用APPLY函数。这是一个最简单的解决方案。您也可以使用WITH运算符。但那看起来更难。

SELECT p.id, p.text, CHILD_ROW.ANY_COLLUMN
FROM parent p
OUTER APPLY (SELECT TOP 1 child.ANY_COLLUMN
                  FROM child
                  WHERE child.parent = p.id
                  ORDER BY child.feature DESC 
                  ) CHILD_ROW

一对多查询选择每个父母的所有父母和单个顶级孩子

5 个答案: