Du kan prova detta:
WITH got_analytics AS
(
SELECT ssn, gpa
, ROW_NUMBER () OVER (ORDER BY gpa DESC) AS r_num
, COUNT (*) OVER () AS n_rows
FROM student_gpa
)
SELECT ssn, gpa
FROM got_analytics
WHERE r_num <= ROUND (n_rows * 12/*insert here your n%*/ / 100)
ORDER BY gpa DESC