sql query help needed!!!

can someone suggest a solution to this..thanks

I have the following tables

1. tbl_subject has subject_id subject_name 1 Biology 2 Chemistry 3 Maths
2. tbl_school has school_id school_name 1 Arthur high 2 NY high 3 MS high
3. tbl_change has change_id change_desc 1 changed subject 2 left - unknown 3 New enrolment 4 changed school
4. tbl_student_activity has activity_id student_id change_id subject_id_From subject_id_To school_id 1 1000 2 null null null 2 1001 1 1 2 null 3 1002 4 null null 1

I want to query the tbl_student_activity table and it should return the following:
I can only use sql to do this...so PL/SQL is not an option :-(

1000 left - unknown
1001 Changed subject Biology - Chemisty
1002 moved school to Arthur high Received on Sat Apr 15 2000 - 00:00:00 CDT