最近在Codility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：
我为 SQLite 中的 SqlEventDelta 问题写了这个解决方案。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出任何建议吗？
※ 我在 Stackoverflow 中搜索了这个问题，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 代码逻辑和函数。
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
它在我的本地工具（DB Browser for SQLite Version 3.12.2）上运行得很好没有错误。
event_type | value
-----------+-----------
2 | -5
3 | 4
Execution finished without errors.
Result: 2 rows returned in 7ms
但是，在web工具（Codility test editor-SQLite Version 3.11.0）上无法运行，我得到以下错误。
| 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，对于每个已注册多次的 event_type，返回最新（即时间上最近的）和第二个最新值之间的差异。
给定一个具有以下结构的表事件：
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，output 应返回以下行集：
event_type | value
-----------+-----------
2 | -5
3 | 4
谢谢。
我尝试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
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
嗨团队！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
with deltas as (
select distinct event_type,
first_value(value) over (PARTITION by event_type ORDER by time DESC) -
nth_value(value, 2) over (PARTITION by event_type ORDER by time DESC) as delta
from events
)
select * from deltas where delta is not null order by 1;
我在使用 sqlite 时确实遇到了同样的问题。 尝试将以下代码与 PostgreSQL 一起使用
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;
添加另一个涉及自连接的答案 -
-- 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
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
一个子查询的解决方案
WITH diff AS
(SELECT event_type,
value,
LEAD(value) OVER (PARTITION BY event_type
ORDER BY TIME DESC) AS prev
FROM EVENTS
GROUP BY event_type,
value,
time
)
SELECT DISTINCT ON (event_type) event_type,
value - prev
FROM diff
WHERE prev IS NOT NULL;
--在 PostgreSQL 9.4 中
with ct1 as (SELECT
event_type,
value,
time,
rank() over (partition by event_type order by time desc) as rank
from events),
ct2 as (
select event_type, value, rank, lag (value,1) over (order by event_type) as previous_value
from ct1
order by event_type)
select event_type, previous_value - value from ct2
where rank = 2
order by event_type
我的解决方案：
--Get table with rank 1, 2 group by event_type
with t2 as(
select event_type, value, rank from (
select event_type, value,
rank() over(
partition by event_type
order by time desc) as rank,
count(*) over (partition by event_type) as count
from events) as t
where t.rank <= 2 and t.count > 1
)
--Calculate diff using Lead() and filter out null diff with max
select t3.event_type, max(t3.diff) from (
select event_type,
value - lead(value, 1) over (
partition by event_type
order by rank) as diff
from t2) as t3
group by t3.event_type
--1) “极简”老式解决方案（不使用窗口函数）
WITH maxevents AS（从事件中选择 event_type 作为 Et1，max(time) 作为 maxtime
按计数 (1)>1 的 event_type 分组，
防止 AS（选择 event_type 作为 Et2，max(time) 作为 nextime
来自 events JOIN maxevents ON event_type=Et1 AND time
SELECT event_type, v1-v2 作为值
FROM maxeventvalues JOIN preveventvalues ON event_type=et
按 event_type 升序排列
;
--2) 使用窗口函数的现代解决方案
WITH maxevents AS（选择 event_type 作为 Et1，max(time) 作为 maxtime
来自按 event_type 计数 (1)>1) 的事件组
,AllValDifs as (SELECT event_type, value-
LAG(value,1,0) OVER (PARTITION BY event_type ORDER BY time ASC) as ValDif , 时间
FROM events JOIN maxevents ON event_type=et1)
选择不同的事件类型，第一个值（ValDif）
OVER (PARTITION BY event_type ORDER BY time DESC) 来自 AllValDifs
按事件类型 ASC 排序；