C# 如何根据按不同对分组的列值选择行?
我的数据库中有以下信息:C# 如何根据按不同对分组的列值选择行?,c#,sql,linq,C#,Sql,Linq,我的数据库中有以下信息: TO | FROM | DATE | PRICE AAA | BBB | 12/3/2012 | $100 AAA | CCC | 12/3/2010 | $200 AAA | BBB | 10/3/2010 | $450 BBB | CCC | 09/7/2010 | $270 BBB | AAA | 06/9/2011 | $130 AAA | CCC | 12/3/2013 | $176 我想编写一个查询(或者最好是LINQ
TO | FROM | DATE | PRICE
AAA | BBB | 12/3/2012 | $100
AAA | CCC | 12/3/2010 | $200
AAA | BBB | 10/3/2010 | $450
BBB | CCC | 09/7/2010 | $270
BBB | AAA | 06/9/2011 | $130
AAA | CCC | 12/3/2013 | $176
我想编写一个查询(或者最好是LINQ表达式)来选择最近的to-FROM对,因此根据上表,我希望在查询中返回以下数据:
TO | FROM | DATE | PRICE
AAA | BBB | 12/3/2012 | $100
BBB | CCC | 09/7/2010 | $270
BBB | AAA | 06/9/2011 | $130
AAA | CCC | 12/3/2013 | $176
最好的方法是什么。因为您没有提到您的RDBMS,下面的查询适用于大多数RDBMS
SELECT a.*
FROM tableName a
INNER JOIN
(
SELECT To, FROM, MAX(DATE) maxDate
FROM tableName
GROUP BY To, FROM
) b ON a.To = b.To AND
a.FROM = b.FROM AND
a.DATE = b.maxDATE
CTE
和WINDOWS功能
WITH latestRecord AS
(
SELECT [To], [FROM], DATE, Price,
ROW_NUMBER() OVER (Partition BY [TO], [FROM] ORDER BY date DESC) rn
FROM tableName
)
SELECT [To], [FROM], DATE, Price
FROM latestRecord
WHERE rn = 1
SELECT [To], [FROM], DATE, Price
FROM
(
SELECT [To], [FROM], DATE, Price,
ROW_NUMBER() OVER (Partition BY [TO],[FROM] ORDER BY date DESC) rn
FROM tableName
) s
WHERE rn = 1
还有一点需要指出的是,
to
和FROM
是一些保留的关键字,它们必须用转义字符转义,转义字符取决于您使用的RDBMS
。谢谢。我也有同样的想法,只是不确定是否有更简单的方法