How to group previously-denormalized-data from a row - postgresql

Question

I have a table containing courses run by teachers, I want to grab the number of taught days and split these by years and teachers' status.
The table contains the following fields:
id teacher_id course_name course_date course_duration teacher_status
--------------------------------------------------------------------------
1 Teacher_01 Course_AA 2012-02-01 2 volunteer
2 Teacher_02 Course_BB 2012-02-01 7 employee
3 Teacher_03 Course_BB 2013-02-01 7 contractor
4 Teacher_01 Course_AA 2014-02-01 2 paid volunteer
5 Teacher_04 Course_AA 2014-06-01 2 paid volunteer
Teachers may run a course under various statuses: volunteer, paid volunteer, contractor, employee, etc. The status of a given teacher can change through time. The duration of a course is expressed in days.
I can already gather the sum of taught days by teachers, split by status. This is done by
SELECT
teacher_status,
sum(course_duration) AS "Taught days"
FROM
my_table
GROUP BY
teacher_status
;
But data is not normalized and different families of statuses have been mixed. So I want to gather the same info (number of taught days) split:
by 3 statuses: volunteer, paid volunteer, all other statuses,
and by years.
What is expected is:
Year Teacher_status Taught_days
---------------------------------------
2012 volunteer 2
2012 employee 7
2013 contractor 7
2014 paid volunteer 4
I've tried various combinations of aggregate functions, GROUP BY / HAVING / ROLLUP statements but without success. How should I achieve this?

Answer 1

You'll want to select a complex expression and then GROUP BY that, not just by a raw column value. You could either repeat the expression or, in Postgres, also refer to the column alias:
SELECT
EXTRACT(year FROM course_date) as year,
(CASE teacher_status
WHEN 'volunteer' THEN 'volunteer'
WHEN 'paid volunteer' THEN 'paid'
ELSE 'other'
END) AS status,
SUM(course_duration) AS "Taught days"
FROM
my_table
GROUP BY
year,
status;

Answer 2

To get your example result, I have this query
SELECT extract (year from course_date),
teacher_status,
sum(course_duration) AS "Taught days"
FROM
my_table
GROUP BY
extract (year from course_date),
teacher_status;