SQL: transposing a time series table into a start-end time table if an event occur - sql

Question

I am trying to use a select statement to create a view, transposing a table with datetime into a table with records in each row, the start-end time when the consecutive values by time (partition by station) in 'record' field is not 0.
Here is a sample of the initial table.
And how it should look like after transposing.
Can anyone help?

Answer 1

You can use the conditional_change_event analytical function to create a special grouping identifier to split these out in a simple query:
select row_number() over () unique_id,
station,
min(datetime) startdate,
max(datetime) enddate
from (
select t.*, CONDITIONAL_CHANGE_EVENT(decode(recording,0,0,1))
over (partition by station order by datetime) chg
from mytable t
) x
where recording > 0
group by station, chg
order by 1, 2
The decode is just to set up your islands and gaps (where gaps are recording <= 0 and islands are recording > 0). Then the change event on that will generate a new identifier for grouping. Also note that I am grouping on the change event even though it isn't part of the output.

Answer 2

ROW_NUMBER() is the best for partitioning. Next, you can do a self join on the partitioned tables to see if the difference between times is greater than five minutes. I think the best solution is to partition on the rolling sum of the timestamp difference, offset by 5 minutes based on your pattern. If the five minutes is not a regular pattern then there is probably a generalized approach that can be used with the zeroes.
Solution written as a CTE below for easy view creation (it's a slow view though).
WITH partitioned as (
SELECT datetime, station, recording,
ROW_NUMBER() OVER(PARTITION BY station
ORDER BY datetime ASC) rn
FROM table --Not sure what the tablename is
WHERE recording != 0),
diffed as (
SELECT a.datetime, a.station,
DATEDIFF(mi,ISNULL(b.datetime,a.datetime),a.datetime)-5) Difference
--The ISNULL logic is for when a.datetime is the beginning of the block,
--we want a 0
FROM partitioned a
LEFT JOIN partitioned b on a.rn = b.rn + 1 and a.station=b.station
GROUP BY a.datetime,a.station),
cumulative as (
SELECT a.datetime, a.station, SUM(b.difference) offset_grouping
FROM diff a
LEFT JOIN diff b on a.datetime >= b.datetime and a.station = b.station ),
ordered as (SELECT datetime,station,
ROW_NUMBER() OVER(PARTITION BY station,offset_grouping ORDER BY datetime asc) starter,
ROW_NUMBER() OVER(PARTITION BY station,offset_grouping ORDER BY datetime desc) ender
FROM cumulative)
SELECT ROW_NUMBER() OVER(ORDER BY a.datetime) unique_id,a.station,a.datetime startdate, b.datetime enddate
FROM ordered a
JOIN ordered b on a.starter = b.ender and a.station=b.station and a.starter=1
This is the only solution I can think of but again, it's slow depending on the amount of data you have.