Question

I have a table that looks like the following:

eventId    activity    timestamp
1          0           2012-10-22 20:10:00
2          0           2012-10-22 20:10:20
3          1           2012-10-22 20:11:25
4          1           2012-10-22 20:12:20
5          1           2012-10-22 20:12:22
6          0           2012-10-22 20:12:30  <--
7          1           2012-10-22 20:12:25  <--
8          0           2012-10-22 20:14:46
9          0           2012-10-22 20:14:48
10         1           2012-10-22 20:15:45
11         0           2012-10-22 20:16:00
12         0           2012-10-22 20:17:00
13         0           2012-10-22 20:17:13

I would like to delete every row that has an activity of 0 and is not chronologically next to a row with activity 1. So from this example I would delete rows with eventID 1, 8, 12, and 13. It is possible for events to be inserted into the table asynchronously, as shown by row 6 and 7.

I know I can do this in a loop by checking each individual row and issuing a query to delete it if it matches my criteria, but that is very inefficient. I was wondering if it is possible to do this all in one query.

It seems like if I could do something similar to

delete from mytable
where activity = 0
and (rownumber()+1).activity = 0
and (rownumber()-1).activity = 0
order by timestamp

this would be simple, but I don’t think such functionality is possible.

Answer 1

Here is how you can select all the records you want to keep:

select m.eventId
from (
  select t1.eventId, 
      max(t2.timestamp) as previousTime,
      min(t3.timestamp) as nextTime
  from mytable t1
  left outer join mytable t2 on t1.eventId <> t2.eventId and t2.timestamp < t1.timestamp
  left outer join mytable t3 on t1.eventId <> t3.eventId and t3.timestamp > t1.timestamp
  group by t1.eventId
) m
left outer join mytable tb on m.previousTime = tb.timestamp
left outer join mytable ta on m.nextTime = ta.timestamp
where tb.activity = 1
    or ta.activity = 1

SQL Fiddle Example

You can then do something like:

delete from mytable
where mytable.eventId not in ( ... ) <--above query