Which Courses Are Hardest?

Students at Fairhaven University have been complaining that some courses are marked far harder than others, and the curriculum committee wants evidence before it decides whether anything needs changing. The measure it has agreed on is the mean mark each course awards.

The marks live on the sittings, which identify a course only by number, so the committee's report has to carry titles — the committee will not read a table of course ids. Sittings with no mark entered yet contribute nothing to a course's mean.

enrollments — one row per sitting: one student taking one course in one term. student_id points at a row in students and course_id at a row in courses. grade is the mark for that sitting, on the same 0-4 scale, and is NULL when no mark has been entered yet. semester is the term the sitting belongs to.

courses — one row per course the university runs. credits is how much the course counts towards a degree.

Both tables already exist in the database — there is nothing to create or load.

Task: Write a query that returns one row per course that has at least one sitting on record, with the course's title and its mean mark rounded to two decimal places as avg_grade. Toughest course first — that is, lowest mean at the top — and if two courses share a mean, the one whose title comes first alphabetically goes above the other.

Example output

Shape only — these courses and means are invented: