Complicated Crosstabbing in PostgreSQL involving Overlapping Dates - sql

Question

I am trying to query the following data:
Student_ID Site Start End Primary_or_Secondary
1 A 1/1/19 2/28/19 Primary
1 B 2/1/19 6/30/19 Secondary
1 C 3/1/19 6/30/19 Primary
and get a result that looks like the following:
Student_ID Primary Secondary Start End
1 A null 1/1/19 1/31/19
1 A B 2/1/19 2/28/19
1 C B 3/1/19 6/30/19
So basically, a site can be primary or secondary site for a student, and I want to be able to see all the time frames the student are enrolled separately instead of any time frame overlapping.
I have wracked my brain about how I might do this in PostgreSQL, and have even looked at the crosstab function, but the dates are making it hard for my brain :-)
Any help with a query or set of queries, including some CTEs would be really helpful!

Answer 1

This isn't trivial. A mix of crosstab with overlapping and intersecting ranges, plus corner cases (merge same start / end dates) on top. Very hard to solve with set-based operations i.e. pure SQL.
I suggest a procedural solution in PL/pgSQL instead. Should perform nicely, too, as it only needs a single (bitmap-index) scan over the table:
CREATE OR REPLACE FUNCTION f_student_xtab(VARIADIC _student_ids int[])
RETURNS TABLE (
student_id int
, "primary" text
, secondary text
, start_date date
, end_date date) AS
$func$
DECLARE
r record;
BEGIN
student_id := -1; -- init with impossible value
FOR r IN
SELECT t.student_id, t.site, t.primary_or_secondary = 'Primary' AS prim, l.range_end, l.date
FROM tbl t
CROSS JOIN LATERAL (
VALUES (false, t.start_date)
, (true , t.end_date)
) AS l(range_end, date)
WHERE t.student_id = ANY (_student_ids)
ORDER BY t.student_id, l.date, range_end -- start of range first
LOOP
IF r.student_id <> student_id THEN
student_id := r.student_id;
IF r.prim THEN "primary" := r.site;
ELSE secondary := r.site;
END IF;
start_date := r.date;
ELSIF r.range_end THEN
IF r.date < start_date THEN
-- range already reported
IF r.prim THEN "primary" := NULL;
ELSE secondary := NULL;
END IF;
start_date := NULL;
ELSE
end_date := r.date;
RETURN NEXT;
IF r.prim THEN
"primary" := NULL;
IF secondary IS NULL THEN start_date := NULL;
ELSE start_date := r.date + 1;
END IF;
ELSE
secondary := NULL;
IF "primary" IS NULL THEN start_date := NULL;
ELSE start_date := r.date + 1;
END IF;
END IF;
end_date := NULL;
END IF;
ELSE -- range starts
IF r.date > start_date THEN
-- range already running
end_date := r.date - 1;
RETURN NEXT;
END IF;
start_date := r.date;
end_date := NULL;
IF r.prim THEN "primary" := r.site;
ELSE secondary := r.site;
END IF;
END IF;
END LOOP;
END
$func$ LANGUAGE plpgsql;
Call:
SELECT * FROM f_student_xtab(1,2,3);
Or:
SELECT * FROM f_student_xtab(VARIADIC '{1,2,3}');
db<>fiddle here - with extended test case
About VARIADIC:
Pass multiple values in single parameter
Return rows matching elements of input array in plpgsql function