Codility SqlEventsDelta（计算每种事件类型的最新值和第二最新值之间的差异）

Question

最近在Codility中练习代码练习。在这里你可以找到问题，它在练习 6 - SQL 部分。刚开始测试看问题描述！ SqlEventsDelta

问题定义：

我在 SQLite 中针对 SqlEventDelta 问题编写了此解决方案。它在本地工具中工作正常但是，它在网络工具中不起作用。

任何人都可以就如何解决这个问题提供任何建议吗？

※ 我在 Whosebug 上搜索了这个问题，我知道比我自己的方法更好的代码。 但是，如果可能的话，我想使用我自己的 SQLite 代码逻辑和功能。

WITH cte1 AS
(
    SELECT *, CASE WHEN e2.event_type = e2.prev THEN 0 
                 WHEN e2.event_type = e2.next THEN 0 
                 ELSE 1 END AS grp
    FROM (SELECT *, LAG(e1.event_type) OVER(ORDER BY (SELECT 1)) AS prev , LEAD(e1.event_type) OVER(ORDER BY (SELECT 1)) AS next FROM events e1) e2
)
,cte2 AS 
(
    SELECT cte1.event_type, cte1.time, cte1.grp, cte1.value - LAG(cte1.value) OVER(ORDER BY cte1.event_type, cte1.time) AS value 
    FROM cte1 
    WHERE cte1.grp = 0 
    ORDER BY cte1.event_type, cte1.time
)

SELECT c2.event_type, c2.value 
FROM cte2 c2
WHERE (c2.event_type, c2.time) IN (
    SELECT c2.event_type, MAX(c2.time) AS time 
    FROM cte2 c2 
    GROUP BY c2.event_type)
GROUP BY c2.event_type
ORDER BY c2.event_type, c2.time

它运行在我的 本地工具（DB Browser for SQLite Version 3.12.2） 上没问题。

event_type | value
-----------+-----------
2          | -5
3          | 4

Execution finished without errors.
Result: 2 rows returned in 7ms

但是，在 web 工具（Codility 测试编辑器-SQLite 版本 3.11.0） 上不能运行我得到了以下错误。

| Compilation successful.

| Example test:   (example test)
| Output (stderr):
| error on query: ...
| ...
| ...,
| details: near "(": syntax error
| RUNTIME ERROR (tested program terminated with exit code 1)

Detected some errors.

SqlEventDelta 问题：

编写一个 SQL 查询，对于每个已注册超过一次的 event_type，return 是最新（即最近的时间）之间的差异) 和第二个最新值。

table 应按 event_type 排序（升序）。
行集中列的名称无关紧要，但它们的顺序很重要。

给定一个 table 事件 具有以下结构：

create table events (
       event_type integer not null,
       value integer not null,
       time timestamp not null,
       unique(event_type, time)
   );

例如，给定以下 数据：

event_type | value      | time
-----------+------------+--------------------
2          | 5          | 2015-05-09 12:42:00
4          | -42        | 2015-05-09 13:19:57
2          | 2          | 2015-05-09 14:48:30
2          | 7          | 2015-05-09 12:54:39
3          | 16         | 2015-05-09 13:19:57
3          | 20         | 2015-05-09 15:01:09

鉴于上述数据，输出应该return 以下行集：

event_type | value
-----------+-----------
2          | -5
3          | 4

谢谢。

Answer 1

我试图使用一种有点幼稚的方法。我知道由于有很多子查询，这对性能非常不利，但这里的问题是 PostgreSQL 的“DISTINCT ON”，但是我得到了 100%

希望你喜欢！

select distinct on (event_type) event_type, result * -1
from (select event_type, value, lead(value) over (order by event_type) - value result
      from (select *
            from events
            where event_type in (select event_type
                                 from events
                                 group by event_type
                                 having count(event_type) >= 2)
            order by event_type, time desc) a) b

Answer 2

我在使用sqlite时遇到了同样的问题。尝试在 PostgreSQL

中使用以下代码

with data as (select 
e.event_type,
e.value,
e.time,
lead(e.value,1) over (PARTITION by e.event_type order by e.event_type,e.time asc) as next_val,
lag (e.value,1) over (PARTITION by e.event_type order by e.event_type,e.time asc) as prev_val
from events e)
select distinct d.event_type, (d.value-d.prev_val) as diff
from 
events e,data d
where e.event_type = d.event_type
and d.next_val is null
and e.event_type in ( SELECT event_type
                        from data 
                        group by 
                        event_type
                        having count(1) > 1)
order by 1;

Answer 3

添加另一个涉及自连接的答案 -

PostgreSQL

-- write your code in PostgreSQL 9.4

WITH TotalRowCount AS (
    SELECT
        event_type,
        COUNT(*) as row_count
    FROM events
    GROUP BY 1
),

RankedEventType AS (
    SELECT
        event_type,
        value,
        ROW_NUMBER() OVER(PARTITION BY event_type ORDER BY time) as row_num
    FROM events
)


SELECT
    a.event_type,
    a.value - b.value as value
FROM RankedEventType a
INNER JOIN TotalRowCount c
    ON a.event_type = c.event_type
INNER JOIN RankedEventType b
    ON a.event_type = b.event_type
WHERE 1 = 1
AND a.row_num = c.row_count
AND b.row_num = c.row_count - 1
ORDER BY 1

Answer 4

没有嵌套查询，得到 100%

with data as (
with count as (select event_type
                                 from events
                                 group by event_type
                                 having count(event_type) >= 2)
select e.event_type , e.value, e.time from events as e inner join count as r on e.event_type=r.event_type  order by e.event_type, e.time desc                               
)
select distinct on (event_type) event_type,
           value - (LEAD(value) over (order by event_type))  result from data

Answer 5

with data as (SELECT a.event_type, a.value, a.time,
 --Produce a virtual table that stores the next and previous values for each event_type.
LEAD(a.value,1) over (PARTITION by a.event_type ORDER by 'event_type', 'time' DESC) as recent_val,
LAG(a.value,1) over (PARTITION by a.event_type ORDER by 'event_type', 'time' DESC) as penult_val
   
    from events a
    
    JOIN (SELECT event_type 
            from events --Filter the initial dataset for duplicates. Store in correct order
                group by event_type HAVING COUNT(*) > 1 
                    ORDER by event_type) b
        
        on a.event_type = b.event_type) --Compare the virtual table to the filtered dataset

SELECT event_type, ("value"-"penult_val") as diff --Perform the desired arithematic
    from data 
    where recent_val is NULL --Filter for the most recent value

大家好！这是我的答案。它主要是上面答案的糊涂集团，但它读起来更简单，并且针对上下文进行了评论。作为新手，希望对其他新手有所帮助

Codility SqlEventsDelta（计算每种事件类型的最新值和第二最新值之间的差异）

Codility SqlEventsDelta (Compute the difference between the latest and the second latest value for each event type)

sql

sqlite

lag

lead

over-clause

PostgreSQL