SELECT DISTINCT在一列上,返回多个其他列(SQL Server)

Question

我正在尝试编写一个查询,从每个独特设备的GPSReport表返回最新的GPS位置.表中有50个设备,所以我只想要返回50行.

这是我到目前为止(不工作)

SELECT TOP(SELECT COUNT(DISTINCT device_serial) FROM GPSReport) * FROM GPSReport AS G1
RIGHT JOIN
(SELECT DISTINCT device_serial FROM GPSReport) AS G2
ON G2.device_serial = G1.device_serial
ORDER BY G2.device_serial, G1.datetime DESC

这将返回50行,但不会为每个device_serial返回唯一的行.它返回第一个设备的所有报告,然后返回第二个设备的所有报告等.

我想在一个查询中做什么？

Answer 1

WITH DEDUPE AS (
    SELECT  *
          , ROW_NUMBER() OVER ( PARTITION BY what_you_want_for_distinct ORDER BY what_you_want_for_distinct) AS OCCURENCE
    FROM tablename
    )
SELECT  * FROM DEDUPE
WHERE
OCCURENCE = 1 

Answer 2

SELECT * FROM
GPSReport AS G1
JOIN (SELECT device_serial, max(datetime) as mostrecent 
      FROM GPSReport group by device_serial) AS G2
ON G2.device_serial = G1.device_serial and g2.mostrecent = g1.datetime
ORDER BY G1.device_serial