how to integrate electrical currents in sqlite - sql

Question

I could solve the following problem in PHP, but I wonder if it could be done in SQLite.
The simplified version looks like this: I have a simple electrical circuit. I can switch on and off a red, a green and a blue light independently. I record the timing in seconds and electrical current in ampere for each light in a table as follows:
| Lamp | On | Off | Current |
|-------|----|:---:|--------:|
| red | 2 | 14 | 3 |
| green | 5 | 8 | 8 |
| blue | 6 | 10 | 2 |
As you can see, they overlap. If I want to integrate the current properly (to calculate the energy consumption), I have to transform this table to a new one, which adds the electrical currents. I get the following table (manually) with adapted timing:
| T1 | T2 | Sum(Current) | Comment |
|:--:|:--:|-------------:|:--------------:|
| 2 | 5 | 3 | red |
| 5 | 6 | 11 | red+green |
| 6 | 8 | 13 | red+green+blue |
| 8 | 10 | 5 | red+blue |
| 10 | 14 | 3 | red |
Any ideas if sqlite can do that? Perhaps by creating interim tables?

Answer 1

It's fairly complex, but I was able to do it with a couple of views:
create table elec (lamp char(10),on_tm int,off_tm int,current int);
insert into elec values
('red',2,14,3),
('green',5,8,8),
('blue',6,10,2);
create view all_tms as
select distinct on_tm
from elec
union
select distinct off_tm
from elec;
create view all_periods as
select t1.on_tm,
(select min(t2.on_tm)
from all_tms t2
where t2.on_tm > t1.on_tm) off_tm
from all_tms t1
select
all_periods.on_tm,
all_periods.off_tm,
sum(case when elec.on_tm <= all_periods.on_tm
and elec.off_tm >= all_periods.off_tm
then elec.current
else 0
end) total_current,
group_concat(case when elec.on_tm <= all_periods.on_tm
and elec.off_tm >= all_periods.off_tm
then elec.lamp
end) lamps
from
all_periods,
elec
group by
all_periods.on_tm,
all_periods.off_tm
The views combine all of the start/stop times into distinct blocks as you have in your output (2-5,5-6, etc.).
The final SELECT evaluates each row from the original table against each time block. If the lamp was on (start time is before the start of the evaluation time, and stop time is after the end of the evaluation time), then its current is counted.

Answer 2

This assumes a sufficiently recent SQLite version; with earlier versions, you would have to replace the common table expressions with temporary views:
WITH all_times(T)
AS (SELECT "On" FROM MyTable
UNION
SELECT Off FROM MyTable),
intervals(T1, T2)
AS (SELECT T,
(SELECT min(T)
FROM all_times AS next_time
WHERE next_time.T > all_times.T) AS T2
FROM all_times
WHERE T2 IS NOT NULL)
SELECT T1,
T2,
(SELECT sum(Current)
FROM MyTable
WHERE T1 >= "On" AND T2 <= Off) AS Current_Sum,
(SELECT group_concat(lamp, '+')
FROM MyTable
WHERE T1 >= "On" AND T2 <= Off) AS Comment
FROM intervals
ORDER BY T1