/* Display instructors that are also students */
/* INTERSECT the intructor data with the student data */

SELECT ssn, fname, lname
from   instructor
INTERSECT
SELECT ssn, fname, lname
from   student;



/* Display ssn of all instructors that are currently teaching a class */
/* INTERSECT the intructor ssn with the class inst_ssn */

SELECT ssn
from   instructor
INTERSECT
SELECT inst_ssn
from   class;



/* You can simulate an INTERSECT by using an inner join */
/* Display ssn of all instructors that are currently teaching a class */

SELECT DISTINCT ssn 
from   instructor, class
where  ssn = inst_ssn;