Postgres - rows to columns - sql

Question

What i have
Query:
SELECT u.firstname,u.lastname, u.institution as department, u.department as company, c.shortname,
to_char(to_timestamp(p.timecompleted)::date,'YYYY-MM-DD') AS completed
FROM mdl_course_completions AS p
JOIN mdl_course AS c ON p.course = c.id
JOIN mdl_user AS u ON p.userid = u.id
WHERE c.enablecompletion = 1 AND u.firstname is NOT NULL AND p.timecompleted is NOT NULL
ORDER BY u.firstname
Results:
firstname lastname department company course completed
u1 u1 x x c1 date
u1 u1 x x c2 date
What i need
I need to be able to transport course to columns. Resulting in something similar to this:
firstname lastname department company c1 c2
u1 u1 x x date date
u2 u2 x x date date
I have tried using crosstab, but I am not skilled enough on SQL. Could someone please help?
EDIT: the number of courses are in the hundreds, so it needs to be dynamic.
(Also: English is not my first language, so please excuse any unclarities).

Answer 1

This is not what you request, but maybe be useful for you. Last column is an array with the structure: {'course_name: course_completed', ...}.
SELECT u.firstname,u.lastname, u.institution as department, u.department as company
array_agg(c.shortname || ': ' || to_char(to_timestamp(p.timecompleted)::date,'YYYY-MM-DD')) AS courses_completed
FROM mdl_course_completions AS p
JOIN mdl_course AS c ON p.course = c.id
JOIN mdl_user AS u ON p.userid = u.id
WHERE c.enablecompletion = 1 AND u.firstname is NOT NULL AND p.timecompleted is NOT NULL
GROUP BY u.firstname,u.lastname, u.institution, u.department
ORDER BY u.firstname
output will be:
firstname lastname department company courses_completed
u1 u1 x x {"c1: date", "c2: date"}
u2 u2 x x {"c1: date", "c2: date"}
Other option is to use crosstab function (like you said): Postgresql crosstab documentation. You have to install tablefunc module to use crosstab. Read the documentation and if you think it could be useful for you I could help you to write the query (but I will need to know the tables structure and some example data).

Answer 2

You can use conditional aggregate, for example by using the FILTER clause:
SELECT
firstname, lastname, department, company,
MAX(completed) FILTER (WHERE course = 'c1') as c1,
MAX(completed) FILTER (WHERE course = 'c2') as c2
FROM
-- <joins>
GROUP BY firstname, lastname, department, company
You need to group by the columns which are part of the same group. Then you can execute any aggregation function you like on the columns you want to pivot. With the appended FILTER clause you can filter all records which should be recognized for the aggregate.