Question

通过语句按选择分区，我很难理解这一点。我读到了有关但不了解。使用的好处是什么？

SELECT MIN(Field) OVER (PARTITION BY OtherField) as Value
FROM MYTABLE

您能否简要描述一下，何时使用？为什么？写这样的东西有什么好处和成就目标

Answer 1

OVER (PARTITION BY OtherField)是窗口函数。此外，主要思想是按分区分组，而不会减少所选表行的数量。

通常，窗口功能将比联接/聚合解决方案更快。这是一个非常简单的情况，因此性能可能基本相同。

让我展示一个例子：

CREATE TABLE dbo.Duration     (startDate datetime2, endDate datetime2);

INSERT INTO dbo.Duration (startDate, endDate)  VALUES ('2007-05-06 16:42:09', '2007-05-07 11:10:08');  
INSERT INTO dbo.Duration (startDate, endDate)  VALUES ('2007-05-07 08:30:09', '2007-05-07 12:12:43');  
INSERT INTO dbo.Duration (startDate, endDate)  VALUES ('2007-05-07 11:35:09', '2007-05-07 17:13:39');  
INSERT INTO dbo.Duration (startDate, endDate)  VALUES ('2007-05-07 08:30:10', '2007-05-07 12:12:43');  
INSERT INTO dbo.Duration (startDate, endDate)  VALUES ('2007-05-07 08:30:11', '2007-05-07 12:12:43');  
INSERT INTO dbo.Duration (startDate, endDate)  VALUES ('2007-05-07 08:30:12', '2007-05-07 12:12:43');

现在，我们可以按字段EndDate进行分区，并获取startDate MIN() EndDate`的in the partition of：

SELECT 
MIN(dr.startDate)  OVER (PARTITION BY dr.endDate ) MinDateByPartitionOfEndDate
,  dr.endDate
FROM dbo.Duration dr

请特别注意2007-05-07 12:12:43及其值2007-05-07 08:30:09.0000000。因此我们没有减少查询结果，但是通过所需字段获得了每个分区的MIN值。

输出：

MinDateByPartitionOfEndDate    endDate
2007-05-06 16:42:09.0000000    2007-05-07 11:10:08.0000000
2007-05-07 08:30:09.0000000    2007-05-07 12:12:43.0000000
2007-05-07 08:30:09.0000000    2007-05-07 12:12:43.0000000
2007-05-07 08:30:09.0000000    2007-05-07 12:12:43.0000000
2007-05-07 08:30:09.0000000    2007-05-07 12:12:43.0000000
2007-05-07 11:35:09.0000000    2007-05-07 17:13:39.0000000
2007-05-07 11:35:09.0000000    2007-05-07 17:13:39.0000000

然后通过GROUP BY（减少查询结果）查询来了解GROUP BY和PARTITION BY之间的主要区别：

SELECT 
MIN(dr.startDate) MinDateByGroupBy
,  dr.endDate
FROM dbo.Duration dr
GROUP BY dr.endDate

输出：

MinDateByGroupBy                    endDate
2007-05-06 16:42:09.0000000     2007-05-07 11:10:08.0000000
2007-05-07 08:30:09.0000000     2007-05-07 12:12:43.0000000
2007-05-07 11:35:09.0000000     2007-05-07 17:13:39.0000000

SQL Server查询选择分区上的最小值

1 个答案: