SQL -- computing end dates from a given start date with arbitrary breaks - sql

Question

I have a table of 'semesters' of variable lengths with variable breaks in between them with a constraint such that a 'start_date' is always greater than the previous 'end_date':
id start_date end_date
-----------------------------
1 2012-10-01 2012-12-20
2 2013-01-05 2013-03-28
3 2013-04-05 2013-06-29
4 2013-07-10 2013-09-20
And a table of students as follows, where a start date may occur at any time within a given semester:
id start_date n_weeks
-------------------------
1 2012-11-15 25
2 2013-02-12 8
3 2013-03-02 12
I am attempting to compute an 'end_date' by joining the 'students' on 'semesters' which takes into account the variable-length breaks in-between semesters.
I can draw in the previous semester's end date (ie from the previous row's end_date) and by subtraction find the number of days in-between semesters using the following:
SELECT start_date
, end_date
, lag(end_date) OVER () AS prev_end_date
, start_date - lag(end_date) OVER () AS days_break
FROM terms
ORDER BY start_date;
Clearly, if there were to be only two terms, it would simply be a matter of adding the 'break' in days (perhaps, cast to 'weeks') -- and thereby extend the 'end_date' by that same period of time.
But should 'n_weeks' for a given student span more than one term, how could such a query be structured ?
Been banging my head against a wall for the last couple of days and I'd be immensely grateful for any help anyone would be able to offer....
Many thanks.

Answer 1

Rather than just looking at the lengths of semesters or the gaps between them, you could generate a list of all the dates that are within a semester using generate_series(), like this:
SELECT
row_number() OVER () as day_number,
day
FROM
(
SELECT
generate_series(start_date, end_date, '1 day') as day
FROM
semesters
) as day_series
ORDER BY
day
(SQLFiddle demo)
This assigns each day that is during a semester an arbitrary but sequential "day number", skipping out all the gaps between semesters.
You can then use this as a sub-query/CTE JOINed to your table of students: first find the "day number" of their start date, then add 7 * n_weeks to find the "day number" of their end date, and finally join back to find the actual date for that "day number".
This assumes that there is no special handling needed for partial weeks - i.e. if n_weeks is 4, the student must be enrolled for 28 days which are within the duration of a semeseter. The approach could be adapted to measure weeks (pass 1 week as the last argument to generate_series()), with the additional step of finding which week the student's start_date falls into.
Here's a complete query (SQLFiddle demo here):
WITH semester_days AS
(
SELECT
semester_id,
row_number() OVER () as day_number,
day_date::date
FROM
(
SELECT
id as semester_id,
generate_series(start_date, end_date, '1 day') as day_date
FROM
semesters
) as day_series
ORDER BY
day_date
)
SELECT
S.id as student_id,
S.start_date,
SD_start.semester_id as start_semester_id,
S.n_weeks,
SD_end.day_date as end_date,
SD_end.semester_id as end_semester_id
FROM
students as S
JOIN
semester_days as SD_start
On SD_start.day_date = S.start_date
JOIN
semester_days as SD_end
On SD_end.day_number = SD_start.day_number + (7 * S.n_weeks)
ORDER BY
S.start_date