Updating a row based on values from a previous row in MySQL 8

Question

I have a table similar to this:

+---------------+---------------------+---------------------+-----------------------+
| event_type_id |     start_time      |      end_time       | hours_from_last_event |
+---------------+---------------------+---------------------+-----------------------+
|             1 | 2019-06-07 0:00:00  | 2019-06-07 22:00:00 |                       |
|             1 | 2019-06-09 21:00:00 | 2019-06-09 23:00:00 |                       |
|             1 | 2019-06-10 16:00:00 | 2019-06-10 20:00:00 |                       |
|             2 | 2019-06-05 14:00:00 | 2019-06-05 19:00:00 |                       |
|             2 | 2019-06-07 03:00:00 | 2019-06-07 21:00:00 |                       |
+---------------+---------------------+---------------------+-----------------------+

For each row, I want to set hours_from_last_event by subtracting the end_time from the previous row from the start_time of the current row ONLY when the event_type_id are equal. If there is no previous row for that row, it should be NULL .

So the above table would become:

+---------------+---------------------+---------------------+-----------------------+
| event_type_id |     start_time      |      end_time       | hours_from_last_event |
+---------------+---------------------+---------------------+-----------------------+
|             1 | 2019-06-07 0:00:00  | 2019-06-07 22:00:00 | NULL                  |
|             1 | 2019-06-09 21:00:00 | 2019-06-09 23:00:00 | 47                    |
|             1 | 2019-06-10 16:00:00 | 2019-06-10 20:00:00 | 17                    |
|             2 | 2019-06-05 14:00:00 | 2019-06-05 19:00:00 | NULL                  |
|             2 | 2019-06-07 03:00:00 | 2019-06-07 21:00:00 | 32                    |
+---------------+---------------------+---------------------+-----------------------+

We can assume that an id column exists to uniquely identify each row.

I already know I can use the TIMEDIFF() function to find the difference in hours, but how should I go about doing this in MySQL?

Answer 1

You can just use lag() for this purpose:

select t.*,
      timestampdiff(hour, lag(end_time) over (partition by event_type_id orde rby start_time), start_time) as diff
from t;

You cannot use this in an update though . . . except through a join :

update t join
       (select t.*,
               timestampdiff(hour, lag(end_time) over (partition by event_type_id order by start_time), start_time) as diff
        from t
       ) tt
       on tt.event_type_id = t.event_type_id and tt.start_time = t.start_time
    set t.hours_from_last_event = tt.diff;