計算記錄之間的時間？

Question

我有一個與datestamps事件表，我想顯示事件之間的時間間隔。舉例來說，如果我有

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

我想編寫一個簡單的查詢，讓我

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 

最後一列的格式並不重要 - 它可以是毫秒，秒，東西人類可讀，無論簡單。另外請注意，實際的數據有適當的日期，我只是懶得輸入日期。

重要的是，在記錄中沒有指向「a」作爲前一個事件的「b」，所以像in this question這樣的解決方案不起作用 - 我需要一些語法來選擇所有記錄加入最近的較舊記錄。

我已經試過看起來像

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; 

，但它只是似乎掛起 - 我讓它運行超過一分鐘。每this question，我試圖

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

這實際工作，但需要在我的表超過14秒，一萬個事件，即使我只選擇10個記錄，所以一定出事了。

Answer 1

嘗試使用

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

要選擇最後10個事件只是使用子查詢與LIMIT而不僅僅是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最近得到正確的第一行（不爲NULL since_last場）應先在內部子查詢LIMIT 11（10 + 1），然後在外部查詢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