Suppose we have a stream of tuples with the schema grades(university, courseid, studentid, grade) assume universities are unique, but a courseid is unique only within a uni- versity (i. e., different universities may have different courses with the same id, e. g., "cs101") and likewise, studentid’s are unique only within a university (different universities may assign the same id to different students). suppose we want to answer certain queries approximately from a 1/20th sample of the data. for each of the queries below, indicate how you would construct the sample. that is, tell what the key attributes should be. that is, tell what the key attributes should be.
(a) estimate the average number of courses per university.
(b) estimate the fraction of students who have a gpa of 3.7 or more.
explain briefly but clearly how you will create the sample and why.