CREATE TABLE grades (name TEXT, class INTEGER, grade INTEGER);

INSERT INTO grades (name, class, grade) VALUES
    ("John", 1, 97), ("Eric", 1, 88), ("Jessica", 1, 98), ("Mike", 1, 82), ("Jeff", 1, NULL),
    ("Ben", 2, 93), ("Andrew", 2, 82), ("Jason", 2, 87), ("Carol", 2, 99), ("Fred", 2, 79);

SELECT class, AVG(grade)
FROM grades
GROUP BY class
HAVING AVG(grade) > 90;