T-SQL Start and end date times from a single column

Question

I have a single table of on/off events for different assets. I need to get a list of start & stop event times without using a cursor.

Source:

Item          


        

Answer 1

Here's the solution, with proof of concept for everyone to verify.

I noticed that the Off event for C was marked as a new On. I fixed that, but that also led me to coding a solution that would allow having an event that had started but not finished, so I included an open ended event D.

Additionally, my solution works with overlapping periods.

declare @YourTable table (Item varchar(10), 
                          EventDate datetime, 
                          Event varchar(10))

insert into @YourTable values
('A',         '2011-10-03 00:01:00',     'On'),
('B',         '2011-10-03 00:01:00',     'On'),
('A',         '2011-10-03 00:02:00',     'Off'),
('C',         '2011-10-03 00:01:00',     'On'),
('B',         '2011-10-03 00:02:00',     'Off'),
('A',         '2011-10-03 00:02:02',     'On'),  
('C',         '2011-10-03 00:02:05',     'Off'),
('A',         '2011-10-03 00:02:07',     'Off'),
('D',         '2011-10-03 00:02:02',     'On')  

select tOn.Item, tOn.EventDate Start, tOff.EventDate [End]
from (
select Item, EventDate, 
       ROW_NUMBER() Over(Partition by Item order by EventDate) EventID
from @YourTable where Event = 'On'
) tOn
LEFT JOIN (
select Item, EventDate, 
       ROW_NUMBER() Over(Partition by Item order by EventDate) EventID
from @YourTable where Event = 'Off'
) tOff
on (tOn.Item = tOff.Item and tOn.EventID = tOff.EventID)

Explained

We divide the data set in 2: On events and Off events. Each containing a numbered row which restarts when Item changes.

Basically, we have a first in first out: The first On will be closed by the first Off, so overlapping periods will be supported by this query given this approach. So each On Event for A will have its Event ID, which will be linked to the correspondent Off Event ID.

Open ended periods will be supported by the LEFT JOIN.