计算记录之间的时间？

Question

我有一个与datestamps事件表，我想显示事件之间的时间间隔。举例来说，如果我有

event | datestamp 
------------------- 
    a | 05:00 
    b | 07:00 
    c | 08:00 
    d | 12:00 
... 

我想编写一个简单的查询，让我

event | datestamp | last | since_last 
----------------------------------------- 
    a | 05:00  | NULL | NULL 
    b | 07:00  | a | 2 hours 
    c | 08:00  | b | 1 hour 
    d | 12:00  | c | 4 hours 

最后一列的格式并不重要 - 它可以是毫秒，秒，东西人类可读，无论简单。另外请注意，实际的数据有适当的日期，我只是懒得输入日期。

重要的是，在记录中没有指向“a”作为前一个事件的“b”，所以像in this question这样的解决方案不起作用 - 我需要一些语法来选择所有记录加入最近的较旧记录。

我已经试过看起来像

SELECT new.event, new.datestamp, old.event AS last, 
    TIMESTAMPDIFF(SECOND, MAX(old.datestamp), new.datestamp) AS since_last 
FROM events AS new, events AS old 
WHERE new.datestamp > old.datestamp 
LIMIT 10; 

，但它只是似乎挂起 - 我让它运行超过一分钟。每this question，我试图

SELECT event, datestamp, 
    (SELECT MAX(old.datestamp) 
    FROM events old 
    WHERE new.datestamp > old.datestamp 
) last_ds 
FROM events new 
LIMIT 10; 

这实际工作，但需要在我的表超过14秒，一万个事件，即使我只选择10个记录，所以一定出事了。

Answer 1

尝试使用

select event,datestamp, 
     @PrevEvent as last, 
     TIMESTAMPDIFF(SECOND, @PrevDate, datestamp) AS since_last, 
     @PrevDate:=datestamp, 
     @PrevEvent:=event 
from events, (select @PrevDate:=null,@PrevEvent:=null) t 
order by datestamp 

SQLFiddle demo

要选择最后10个事件只是使用子查询与LIMIT而不仅仅是events表。

select event,datestamp, 
     @PrevEvent as last, 
     TIMESTAMPDIFF(SECOND, @PrevDate, datestamp) AS since_last, 
     @PrevDate:=datestamp, 
     @PrevEvent:=event 
from (select * 
     from events 
     order by datestamp DESC 
     LIMIT 10 
    ) t1, (select @PrevDate:=null,@PrevEvent:=null) t 
order by datestamp 

UPD：

要在10最近得到正确的第一行（不为NULL since_last场）应先在内部子查询LIMIT 11（10 + 1），然后在外部查询LIMIT 10 OFFSET 1 。

select event,datestamp,last,since_last 
FROM 
(
select event,datestamp, 
     @PrevEvent as last, 
     TIMESTAMPDIFF(SECOND, @PrevDate, datestamp) AS since_last, 
     @PrevDate:=datestamp, 
     @PrevEvent:=event 
from (select * 
     from events 
     order by datestamp DESC 
     LIMIT 11 
    ) t1, (select @PrevDate:=null,@PrevEvent:=null) t 
order by datestamp 
) t3 
LIMIT 10 offset 1 

Answer 2

Select event,datestamp,last,since_last FROM 
(select event,datestamp, 
     @PrevEvent as last, 
     concat((TIMESTAMPDIFF(SECOND, @PrevDate, datestamp))/3600, 
       ' Hours') AS since_last, 
     @PrevDate:=datestamp, 
     @PrevEvent:=event 
from events, (select @PrevDate:=null,@PrevEvent:=null) t 
order by datestamp) abc 

Fiddle