SELECT *FROM student NATURAL FULL OUTER JOIN takes NATURAL FULL OUTER JOIN course
include tuples with null values for the title attribute?
If the course already had a null value for the title attribute.
This is a valid circumstance since there is no NOT NULL domain constraint
in the schema of course relation.
If a student takes a course that is not given by his/her department. (Recall that
dept_name attribute appears in both course relation and student relation).
