Question

说我有一个包含三列的表：c1，c2和c3。我想在每个c1中获得c2的最大值和包含最大值的行的c3的值。

这是一个例子。说MyTable包含以下记录：

c1   c2   c3
---  ---  ---
1    32   12
1    27   15
1    12   21
2    23   33
2    36   49
3    48   17
3    21   50
3    7    25

然后我的查询应该返回

c1   MAX(c2)  c3
---  -------  ---
1    32       12
2    36       49
3    48       17

但是当我查询

SELECT c1, MAX(c2), c3 FROM MyTable GROUP BY c1

我得到了错误：

Column 'MyTable.c3' is invalid in the select list because it is not 
contained in either an aggregate function or the GROUP BY clause.

如何解决此查询？我尝试了子查询，联接等，但无济于事。

Answer 1

我认为where中的相关子查询是一种简单的解决方案：

select t.*
from t
where t.col2 = (select max(t2.col2) from t t2 where t2.col1 = t.col1);

这通常具有非常好的性能特征。更传统的解决方案使用row_number(：

select t.*
from (select t.*,
             row_number() over (partition by col1 order by col2 desc) as seqnum
      from t
     ) t
where seqnum = 1;

如果您有单独的col1值表，那么apply也可以具有良好的性能：

select t.*
from table_col1 c1 cross apply
     (select top (1) t.*
      from t 
      where t.col1 = c1.col1
      order by t.col2 desc
     ) t;

Answer 2

您可以尝试：

SELECT a.c1, a.c2, a.c3 
FROM
MyTable a
INNER JOIN
(SELECT c1, MAX(c2) as 'max_c2' FROM MyTable GROUP BY c1) b
ON
a.c1 = b.c1
AND
a.c2 = b.max_c2

Answer 3

您可以利用OVER()和DECLARE @MyTable TABLE(c1 INT, c2 INT, c3 INT) INSERT INTO @MyTable VALUES (1, 32 , 12), (1, 27 , 15), (1, 12 , 21), (2, 23 , 33), (2, 36 , 49), (3, 48 , 17), (3, 21 , 50), (3, 7 , 25) SELECT * FROM ( SELECT c1 , MAX(c2) OVER(PARTITION BY c1) c2 , CASE WHEN c2 = MAX(c2) OVER(PARTITION BY c1) THEN c3 ELSE NULL END c3 FROM @MyTable ) D WHERE c3 IS NOT NULL子句。

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汇总查询中的非分组列

3 个答案: