A two-credit 10

Problem

Grades go from 0 to 10. Henry has a 10 in Music, an 8 in History, and a 5.5 in Math, and his average is 7.83. The school isn't buying it: Music is worth 2 credits and Math is worth 5. In a weighted average, each grade counts as many times as its course has credits: you add up each grade multiplied by its credits and divide by the total credits. Henry's is 7.15.

For each student with at least one grade entered, show the student_id, the credits already graded, and the weighted average, rounded to 2 decimals, in that order. A course whose grade is still pending adds nothing yet, neither its grade nor its credits.

What it has to return

This is what comes out with the data in the practice database this challenge uses. The column names don’t count, so name yours however you like: what counts is how many there are and the order they come in. Besides this one, the challenge has hidden tests with other data that are revealed when you submit your solution.

You start with this

SELECT g.student_id, g.grade,
  c.credits
FROM grades g
JOIN courses c
  ON g.course_id = c.id