Computing average accessing multiple tables

Question

I have a teaser in creating a list of students and computing their average grade by accessing multiple tables.. can someone help me with this?
I have following tables:
class( class_id, class_name)
student(student_id, student_name, class_id)
exam ( exam_id , class_id , exam_date )
grade( exam_id , student_id, grade )
create a list of students and ther avg grade in their last N ( n can be any number ) tests
Any help in getting this sql will be useful

Answer 1

Hi,
Welcome to the forum!
I'd approach the problem in these steps
(1) Join the tables to show all grades for all students: one row per student per grade
(2) Use the analytic ROW_NUMBER (or RANK, depending on how you want to deal with ties) to see which is the 1st, 2nd, 3rd, ... most recent for each student. (Let's call this number rnum).
(3) In a super-query, compute the average "WHERE rnum <= n".
Break these steps down, if necessary, and test that your query is doing what you want before going on to the next step.
For example, (1) is a lot to do at once. Take baby steps. for example:
(1a) Start with just the grade table. Write a query that gets the relevant onformation from the grade table. When that is working perfectly,
(1b) Add one more table (either exam or student will work). Make sure you're getting all the data you need from these 2 tables.
If you get stuck, post a little sample data (CREATE TABLE and INSERT statements, relevant columns only), your query, and the results you want to get from that data with that query.