获取不同子组中的值计数

Question

我需要删除数据集中的一些行，其中speed 等于零并持续 N 次（假设 N 为 2）。 表demo 的结构如下：

id	car	speed	time
1	foo	0	1
2	foo	0	2
3	foo	0	3
4	foo	1	4
5	foo	1	5
6	foo	0	6
7	bar	0	1
8	bar	0	2
9	bar	5	3
10	bar	5	4
11	bar	5	5
12	bar	5	6

然后我希望使用window_function生成如下表：

id	car	speed	time	lasting
1	foo	0	1	3
2	foo	0	2	3
3	foo	0	3	3
4	foo	1	4	2
5	foo	1	5	2
6	foo	0	6	1
7	bar	0	1	2
8	bar	0	2	2
9	bar	5	3	4
10	bar	5	4	4
11	bar	5	5	4
12	bar	5	6	4

然后我可以使用WHERE NOT (speed = 0 AND lasting > 2)轻松排除这些行

把我试过的代码放在这里，但它没有返回我期望的值，我猜那些FROM (SELECT ... FROM (SELECT ...可能不是解决问题的最佳实践：

SELECT g3.*, count(id) OVER (PARTITION BY car, cumsum ORDER BY id) as num   
  FROM (SELECT g2.*, sum(grp2) OVER (PARTITION BY car ORDER BY id) AS cumsum             
    FROM (SELECT g1.*, (CASE ne0 WHEN 0 THEN 0 ELSE 1 END) AS grp2                            
      FROM (SELECT g.*, speed - lag(speed, 1, 0) OVER (PARTITION BY car) AS ne0              
        FROM (SELECT *, row_number() OVER (PARTITION BY car) AS grp FROM demo) g ) g1 ) g2 ) g3                                                                                       
ORDER BY id;

Answer 1

您可以使用窗口函数 LAG() 检查每行的前一个 speed 值，并使用 SUM() 窗口函数为连续值创建组。
然后使用COUNT()窗口函数可以统计每组的行数，这样就可以过滤掉超过2行的组中speed为0的行：

SELECT id, car, speed, time
FROM (
  SELECT *, COUNT(*) OVER (PARTITION BY car, grp) counter
  FROM (
    SELECT *, SUM(flag::int) OVER (PARTITION BY car ORDER BY time) grp
    FROM (
      SELECT *, speed <> LAG(speed, 1, speed - 1) OVER (PARTITION BY car ORDER BY time) flag
      FROM demo
    ) t  
  ) t
) t
WHERE speed <> 0 OR counter <= 2
ORDER BY id;

请参阅demo。