MYSQL Problem

The relation Employee(EId, DepartmentId, ProjectId, Salary) stores employee information. For example, ('E1', 'D2', 'P3', '90000') indicates that employee with Id 'E1' of department 'D2' with a salary of $90,000 is working on the project 'P3'. An employee belongs to one and only one department but can work on multiple projects, or no project at all. For example, ('E1', 'D2', 'P25', '90000') can also be in the relation. However, an employee always has an unique rank.

(a) Can EId have a null value?

(b) Can DepartmentId have a null value?

(c) Can ProjectId have a null value?

(d) Can Salary have a null value?

(e) Show the candidate keys.

2) It is known that R(A,B,C,D,E) have two candidate keys AB and CDE. How many superkeys are there and what are them?