sql - GROUP BY with MAX(DATE)

Question

我正在尝试在表格中列出每列火车的最新目的地（最大出发时间），例如：

Train    Dest      Time
1        HK        10:00
1        SH        12:00
1        SZ        14:00
2        HK        13:00
2        SH        09:00
2        SZ        07:00

期望的结果应该是：

Train    Dest      Time
1        SZ        14:00
2        HK        13:00

我试过使用

SELECT Train, Dest, MAX(Time)
FROM TrainTable
GROUP BY Train

我收到“ora-00979 不是 GROUP BY 表达式”错误，说我必须在我的 group by 语句中包含“Dest”。但这肯定不是我想要的……

是否可以在一行 SQL 中完成？

Answer 1

SELECT train, dest, time FROM ( 
  SELECT train, dest, time, 
    RANK() OVER (PARTITION BY train ORDER BY time DESC) dest_rank
    FROM traintable
  ) where dest_rank = 1

Answer 2

您不能在结果集中包含未分组的非聚合列。如果一列火车只有一个目的地，那么只需将目的地列添加到您的 group by 子句中，否则您需要重新考虑您的查询。

尝试：

SELECT t.Train, t.Dest, r.MaxTime
FROM (
      SELECT Train, MAX(Time) as MaxTime
      FROM TrainTable
      GROUP BY Train
) r
INNER JOIN TrainTable t
ON t.Train = r.Train AND t.Time = r.MaxTime

Answer 3

这是一个仅使用左连接的示例，我相信它比任何分组方法都更有效：ExchangeCore 博客

SELECT t1.*
FROM TrainTable t1 LEFT JOIN TrainTable t2
ON (t1.Train = t2.Train AND t1.Time < t2.Time)
WHERE t2.Time IS NULL;

Answer 4

另一种解决方案：

select * from traintable
where (train, time) in (select train, max(time) from traintable group by train);

Answer 5

只要没有重复（火车往往一次只到达一个车站）......

select Train, MAX(Time),
      max(Dest) keep (DENSE_RANK LAST ORDER BY Time) max_keep
from TrainTable
GROUP BY Train;

Answer 6

我知道我迟到了，但试试这个...

SELECT 
    `Train`, 
    `Dest`,
    SUBSTRING_INDEX(GROUP_CONCAT(`Time` ORDER BY `Time` DESC), ",", 1) AS `Time`
FROM TrainTable
GROUP BY Train;

Src：组 Concat 文档

编辑：固定的sql语法