Answer Happy

Consider the following schemas of tables in a student registration system. Table prerequisite Table course cid integer title varchar(40) pre integer post integer level integer Table student Table registered sid integer sid integer name varchar(40) cid integer year integer (a) Write a relational algebra expression for the following problem: Find the ID numbers of students who registered for some level 2 course in 2020. [4 marks] (b) Write a relational algebra expression for the following problem: Find the ID numbers of students who registered for only level 2 courses in 2020. Or, to put another way, find the ID numbers of students who registered for courses all of which are at level 2. [4 marks] Hint: You may find the following logical equivalence useful in devising your solution: for all x, P = not (exists x such that not P) (c) Define precisely when a set of tables is said to be in Boyce-Codd normal form. [2 marks]