获得每组的第一行

Question

我想转换这些数据

account; docdate; docnum
17700; 9/11/2015; 1
17700; 9/12/2015; 2
70070; 9/1/2015; 4
70070; 9/2/2015; 6
70070; 9/3/2015; 9

进入这个

account; docdate; docnum
17700; 9/12/2015; 2
70070; 9/3/2015; 9

..对于每个帐户，我想拥有一行最新（= max（docdate））docdate。我已经尝试了cross apply和row_number的不同方法，但无法达到预期效果

Answer 1

使用ROW_NUMBER：

SELCT account, docdate, docnum
FROM (
  SELECT account, docdate, docnum,
         ROW_NUMBER() OVER (PARTITION BY account 
                            ORDER BY docdate DESC) AS rn
  FROM mytable ) AS t
WHERE t.rn = 1 

PARTITION BY account子句创建共享相同account值的行片。 ORDER BY docdate DESC将具有最大docdate值的记录放在其相关切片的顶部。因此rn = 1指向每个docdate分区中具有最大account值的记录。