ER-Model

Consider two tables in a relational database with columns and rows as follows:

Table: Student
ROLL_NO	  NAME	DEPT_ID
   1	  ABC	   1
   2	  DEF	   1
   3	  GHI	   2
   4	  JKL	   3
Table: Department
DEPT_ID	DEPT_NAME
   1	   A
   2	   B
   3	   C 

Roll_no is the primary key of the Student table, Dept_id is the primary key of the Department table and Student.Dept_id is a foreign key from Department.Dept_id.
What will happen if we try to execute the following two SQL statements?

(i)  update Student set Dept_id = Null 
     where Roll_on = 1
(ii) update Department set Dept_id = Null 
     where Dept_id = 1