SQL Duration that value was true - sql-server

Question

Edited: to include sample data
Looking for guidance on a TSQL query.
I have a table that stores readings from a sensor (Amperage). The table basically has a Date/Time and a Value column.
The date/time increments every 5 seconds (a new record is added on 5 second intervals).
I am trying to build a query to determine the duration of time that the value was >X.
Example Data:
http://sqlfiddle.com/#!18/f15c0/1/0
The example data is missing chunks to make it smaller but think you would get the idea.
I am trying to get the first record to the next record that goes above 7. This I would do a datediff to get the duration in seconds from when the data started to that first record over 7. I then need to repeat this but now find when it goes below 7.
This way I can see the cycle time duration.
Think of it as your Fridge. The sensor checks in every 5 seconds and sees that the fridge is off and records that. Eventually the fridge turns on and remains on for a period of time. I am trying to get all those cycle times.
I am trying to use Lead and Lag functions...but just getting lost in regards to pulling the data.
Any help?

Answer 1

declare #val numeric(10,5) = 7.0
select v1.entrydate,
v1.Amps,
case when v1.fl = 1 and v1.lg is null then 1
when v1.lg != v1.fl then 1
else 0
end as fl_new
from (
select v1.entrydate,
v1.Amps,
case when v1.Amps > #val then 1
else 0
end as fl,
lag(case when v1.Amps > #val then 1
else 0
end) over(order by v1.entrydate) as lg
from (
select t.entrydate as entrydate,
t.Amps as Amps
from YourTable t
) v1
) v1
where case when v1.fl = 1 and v1.lg is null then 1
when v1.lg != v1.fl then 1
else 0
end = 1
order by v1.entrydate
And don't forget set YourTable name and #val (which is "X").

Answer 2

Images are blocked at my current location, so I can't see your structure. I'll assume you have the following table (I'll ignore PK and other constraints):
create table reading(
entryDate datetime,
amps int
)
Assuming anything above 3 amps is ON, and you want to compute the duty cycles in seconds, then
declare #threshold int = 3;
with
state as (
select entryDate,
case when amps>#threshold then 'ON' else 'OFF' end state,
lag( case when amps>#threshold then 'ON' else 'OFF' end )
over(order by entryDate) prev_state
from reading
),
transition as (
select entryDate, state
from state
where state <> coalesce(prev_state,'')
)
select entryDate,
state,
dateDiff(
s,
entryDate,
lead(entryDate) over(order by entryDate)
) duration
from transition
order by 1

Answer 3

Not sure how fast it'll be, but if you want to try with LAG?
Here's an example that checks for a difference of X>=2
SELECT entrydate, amps
FROM
(
SELECT
entrydate, amps,
amps - LAG(amps) OVER (ORDER BY entrydate) AS PrevAmpsDiff
FROM YourTable
) q
WHERE ABS(FLOOR(PrevAmpsDiff)) >= 2
ORDER BY entrydate;
A test on rextester here