sql - 分组中第一行和最后一行之间的差异

Question

我从非常复杂的查询中获取了这张表，也就是说，它不可能自己加入。行按时间排序。

type, value, time
+---+----+
| 2 |  2 |
| 2 |  7 |
| 3 | 20 |
| 3 | 16 |
+---+----+ 

我需要计算每个类型分组的第一个值和最后一个值之间的差异，在给定的示例中这将给我

+---+----+
| 2 | -5 |
| 3 | 4  |
+---+----+  

是否可行？

Answer 1

一种方法使用窗口函数。像这样的东西有效:

select distinct type,
       (first_value(value) over (partition by type order by time asc) -
        first_value(value) over (partition by type order by time desc)
       ) as diff        
from t;

不幸的是，Postgres 没有first_value() 聚合函数。

您也可以使用 array_agg() 执行此操作:

select distinct type,
       ((array_agg(value order by time asc))[1] -
        (array_agg(value order by time desc))[1]
       ) as diff        
from t
group by type;