MySQL query add duration to previous record

问题

I like to add event duration to a previous record every time a new record gets added.

This is what I have

ID EventType EventTime EventDuration
-------------------------------------
1  TypeA     10:20     NULL
2  TypeB     09:30     NULL
3  TypeC     08:00     NULL

This is what I want to achieve:

ID EventType EventTime EventDuration
-------------------------------------
1  TypeA     10:20     00:50
2  TypeB     09:30     01:30
3  TypeC     08:00     ...
4  ...       ...

When a new records gets added (with ID, EventType and EventTime), the duration of the previous record (timediff between TypeB and Type A) should be added to the previous record in column EventDuration.

What I have so far is:

SELECT 
   id, EventTime,
   timestampdiff(minute,
      (SELECT EventTime 
       FROM TableName t2
       WHERE t2.id < t1.id ORDER BY t2.id DESC LIMIT 1),EventTime) 
       AS EventDuration
       FROM records t1
       WHERE id = ....<this is where I get stuck, this should be a query that identifies the ID of latest EventTime record>

Any suggestions?

(I am running MySQL 5.6.39)

回答1:

If you are running MySQL 8.0, you can use window functions for this:

update mytable t
inner join (
    select id, timediff(eventTime, lag(eventTime) over(order by eventTime)) event_duration
    from mytable t
) t1 on t1.id = t.id
set t.event_duration = t1.event_duration

If you want to update only the last but 1 record, you can order by and limit in the subquery (or in the outer query):

update mytable t
inner join (
    select id, timediff(eventTime, lag(eventTime) over(order by eventTime)) event_duration
    from mytable t
    order by id desc
    limit 1, 1
) t1 on t1.id = t.id
set t.event_duration = t1.event_duration

In earlier versions, one option is to emulate lag() with a window function:

update mytable t
inner join (
    select 
        id, 
        timediff(
            (select min(eventTime) from mytable t1 where t1.eventTime > t.eventTime),
            eventTime
        ) event_duration
    from mytable t
) t1 on t1.id = t.id
set t.event_duration = t1.event_duration

来源：https://stackoverflow.com/questions/61821143/mysql-query-add-duration-to-previous-record

标签

mysql

sql

time

sql-update

duration