Question

I have a database with trending data, the structure is like this:

[Timestamp, System_State]
2012-01-01 00:00:00, 1
2012-01-01 00:01:00, 1
2012-01-01 00:02:00, 0
2012-01-01 00:03:00, 1
2012-01-01 00:04:00, 1
2012-01-01 00:05:00, 0
...

Now I need to write a SQL query to know when the system starts (system_state changes from 0 to 1) and when it stops (state changes from 1 to 0). Do I have any options beside using cursor?

Thanks.

Answer 1

How about something like this (this is T-SQL, i.e. SQL Server, but may be applicable to MySql too)

declare @t table (eventTime datetime, eventState bit)

insert @t select '2012-01-01 00:00:00', 1
union all select '2012-01-01 00:01:00', 1
union all select '2012-01-01 00:02:00', 0
union all select '2012-01-01 00:03:00', 1
union all select '2012-01-01 00:04:00', 1
union all select '2012-01-01 00:05:00', 0

select *
from
(
    select eventTime, eventState, row_number() over (order by eventTime, eventState)r 
    from @t
) a
inner join
(
    select eventTime, eventState, row_number() over (order by eventTime, eventState)r 
    from @t
) b
on a.r = b.r-1
where a.eventState <> b.eventState

–alteratively:

select *
from @t a
where exists (
    select 1
    from @t b
    where a.eventTime > b.eventTime
    and a.eventState != b.eventState
    and not exists
    (
        select 1
        from @t c
        where a.eventTime > c.eventTime
        and b.eventTime < c.eventTime
    )
)
order by eventTime

–or

select *
from @t a
inner join @t b
    on a.eventTime > b.eventTime
    and a.eventState != b.eventState
left outer join @t c
    on a.eventTime > c.eventTime
    and b.eventTime < c.eventTime
where c.eventState is null
order by a.eventTime