如何根据按不同对分组的列值选择行？

Question

我的数据库中包含以下信息：

 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（最好是LINQ表达式）来选择最新的TO-FROM对，因此基于上表，我希望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

做这个的最好方式是什么。

Answer 1

由于您没有提到RDBMS，因此以下查询适用于大多数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

• SQLFiddle演示链接

但是如果您是RDBMS，则支持CTE和WINDOWS FUNCTION

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

• SQLFiddle演示链接

或WINDOWS FUNCTION ONLY

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

• SQLFiddle演示链接

需要指出的另一点是， TO和FROM是保留关键字，必须使用转义的字符对其进行转义，这取决于您使用的RDBMS 。