How to represent NULL in relational algebra?
There are no NULL values in relational algebra. Here you can use the antijoin operator &#x25B7;:


  πROLLNO, NAME(STUDENT &#x25B7;ENROLLMENT)


that selects all records from STUDENT where the ROLLNO is not in ENROLLMENT.

In SQL there are NULL values. You may use a left outer join and select the rows where the courseid is NULL:

select STUDENT.ROLLNO, STUDENT.NAME
from STUDENT left outer join ENROLLMENT 
    on(STUDENT.ROLLNO=ENROLLMENT.ROLLNO)
where ENROLLMENT.COURSEID is null