Why can't I get the right answer in this SQLzoo tutorial?

Question

Problem 2b goes as follows:

2b. For each subject show the first year that the prize was awarded.

nobel(yr, subject, winner)

My solution was this:
SELECT DISTINCT subject, yr
FROM nobel
ORDER BY yr ASC;

Why isn't this working?

Answer 1

Your answer gets a row for every distinct combination of subject and year.

The correct answer GROUPS BY the subject, and gets the MIN year per subject.

Enough of a clue?

Answer 2

You could do it a different way without using group by or min

select distinct subject, yr from nobel x
  where yr <= all
    (select yr from nobel y
     where y.subject = x.subject)

but its definitely more work.

Answer 3

SELECT subject, MIN(yr)
FROM nobel
GROUP BY subject;

Yay!