如何获取每对列的计数和最新记录？

Question

我有一个 Athena table 有 4 列 (A, B, C, D) 我想找到：

与 A 和 B
同一对 A 和 B 的最近一行的 C 值，其中 D 是时间戳

例如，如果这是输入数据

+---+---+-----+------------+
| A | B |  C  |     D      |
+---+---+-----+------------+
| 1 | 1 | 'a' | 2019-04-04 |
| 1 | 1 | 'b' | 2019-04-03 |
| 1 | 2 | 'c' | 2019-04-02 |
| 1 | 3 | 'd' | 2019-04-01 |
| 2 | 2 | 'e' | 2019-04-03 |
| 2 | 2 | 'f' | 2019-04-04 |
+---+---+-----+------------+

这是期望的输出

+---+---+----------+-------+
| A | B | newest_C | count |
+---+---+----------+-------+
| 1 | 1 | 'a'      |     2 |
| 1 | 2 | 'c'      |     1 |
| 1 | 3 | 'd'      |     1 |
| 2 | 2 | 'f'      |     2 |
+---+---+----------+-------+

我不太擅长查询，我最好的尝试如下：

加入两个子查询，其中一个进行计数，另一个按时间对每一行进行排名。然后在连接中，只有 select 排名最高的行。

WITH t1 AS (
    SELECT A, B, count(*)
    FROM data
    GROUP BY A, B
),
t2 AS (
    SELECT A, B, C, RANK() OVER (PARTITION BY A, B ORDER BY D DESC) AS rank
    FROM data
)
SELECT t1.A, t1.B, t2.newest_C, t1.count
FROM t1 LEFT JOIN t2 ON t1.A = t2.A AND t1.B = t2.B
WHERE rank = 1

Answer 1

这可以使用 Presto window functions 来实现：

SELECT a, b, c AS newest_c, cnt
FROM (
    SELECT 
        t.*,
        COUNT(*)     OVER(PARTITION BY a, b) AS cnt,
        ROW_NUMBER() OVER(PARTITION BY a, b ORDER BY d DESC) AS rn
    FROM mytable t
) x WHERE rn = 1

在子查询中，可以使用window函数统计具有相同(a, b)元组的记录数，并按d降序排列记录。然后，外部查询过滤每个组中的最新记录。

Answer 2

Presto 有一些复杂的聚合函数。所以：

select a, b, count(*) as cnt,
       max_by(c, d)
from t
group by a, b;

max_by()在documentation.

中有解释

Answer 3

Gordon Linoff 的解决方案是可以的。如果您不想使用 max_by:

的另一种选择

SELECT t1.a, t1.b, t1.c, t2.count
FROM data AS t1 
INNER JOIN
  (SELECT a, b, count(*) AS count, max(d) AS d 
  FROM data 
  GROUP BY a,b) AS t2
ON t1.a = t2.a AND t1.b = t2.b AND t1.d = t2.d

这里是a demo!

如何获取每对列的计数和最新记录？

How can I get the count and newest record for each pair of columns?

sql

amazon-athena

presto