SQLite subtract time difference between two tables if there is a match - sqlite

Question

I need some help with a SQLite Query. I have two tables, a table called 'production' and a table called 'pause':
CREATE TABLE production (
date TEXT,
item TEXT,
begin TEXT,
end TEXT
);
CREATE TABLE pause (
date TEXT,
begin TEXT,
end TEXT
);
For every item which is produced, an entry in the table production with the current date, the start time and the end time (two timestamps in the format HH:MM:SS) is created. So let's assume, the production table looks like:
+------------+-------------+------------+----------+
| date | item | begin | end |
+------------+-------------+------------+----------+
| 2013-07-31 | Item 1 | 06:18:00 | 08:03:05 |
| 2013-08-01 | Item 2 | 06:00:03 | 10:10:10 |
| 2013-08-01 | Item 1 | 10:30:15 | 14:20:13 |
| 2013-08-01 | Item 1 | 15:00:10 | 16:00:00 |
| 2013-08-02 | Item 3 | 08:50:00 | 15:00:00 |
+------------+-------------+------------+----------+
The second table also contains a date and a start and an end time. So let's assume, the 'pause' table looks like:
+------------+------------+----------+
| date | begin | end |
+------------+------------+----------+
| 2013-08-01 | 08:00:00 | 08:30:00 |
| 2013-08-01 | 12:00:00 | 13:30:00 |
| 2013-08-02 | 10:00:00 | 10:30:00 |
| 2013-08-02 | 13:00:00 | 14:00:00 |
+------------+------------+----------+
Now I wanna get a table, which contains the time difference between the production begin and end time for every item. If there is a matching entry in the 'pause' table, the pause time should be subtracted.
So basically, the end result should look like:
+------------+------------+-------------------------------------------------+
| date | Item | time difference (in seconds), excluding pause |
+------------+------------+-------------------------------------------------+
| 2013-07-31 | Item 1 | 6305 |
| 2013-08-01 | Item 1 | 12005 |
| 2013-08-01 | Item 2 | 13207 |
| 2013-08-02 | Item 3 | 16800 |
+------------+------------+-------------------------------------------------+
I am not really sure, how I can accomplish it with SQLite. I know that it is possible to do this sort of calculation with Python, but in the end I think it would be better to let the database do the calculations. Maybe someone of you could give me a hint on how to solve this problem. I tried different queries, but I always ended up with different results than I expected.

Answer 1

To convert a time string to the number of seconds, use the strftime function with the %s modifier.
(A time string without a date part will be assumed to have the date 2000-01-01, but this cancels out when computing the differences.)
To compute the pause times for a specific production record, use a correlated subquery; the total aggregate is needed to cope with zero/one/multiple matching pauses.
SELECT date,
item,
sum(strftime('%s', end) - strftime('%s', begin) -
(SELECT total(strftime('%s', end) - strftime('%s', begin))
FROM pause
WHERE pause.date = production.date
AND pause.begin >= production.begin
AND pause.end <= production.end)
) AS seconds
FROM production
GROUP BY date,
item

Answer 2

The best answer I found is:
SELECT
cast(
(
strftime('%s',time_arrived)-strftime('%s',time_departed)
) AS real
)/60/60 AS elapsed
FROM date AS t;
For aditional information check this blog article.