Not In Vs Minus in SELECT statement of Oracle

What is the diff between NOT IN and MINUS. As per the SET theory, both should yield the same results.

Lets take an example : From the scott schema by using the Emp and Dept table, if the following queries were fired to get the list of DEPARTMENTS that do not have any EMPLOYEE, then,

SELECT deptno FROM dept WHERE deptno NOT IN (SELECT deptno FROM emp);

and

SELECT deptno FROM dept
MINUS
SELECT deptno FROM emp;

both queries will return no rows.
--------------------------------------------------------------
Now, lets look at the following data:

DEPARTMENT table got few more rows

DEPTNO	DNAME	LOC
10	ACCOUNTING	NEW YORK
20	RESEARCH	DALLAS
30	SALES	CHICAGO
40	OPERATIONS	BOSTON
50	QUALITY	CHICAGO
60	IT	CHICAGO
70	MARKETING	NEY YORK

EMPLOYEE table
For the first row, employee no 9999, there is no department assigned. Deptno is NULL.

EMPNO	ENAME	JOB	MGR	HIREDATE	SAL	COMM	DEPTNO	STATUS
9999	James	James	7782	23-Jan-82	1300	(null)	(null)	Y
7369	SMITH	CLERK	7902	17-Dec-80	800	(null)	20	Y
7499	ALLEN	SALESMAN	7698	20-Feb-81	1600	300	30	Y
7521	WARD	SALESMAN	7698	22-Feb-81	1250	500	40	Y
7566	JONES	MANAGER	7839	2-Apr-81	2975	(null)	20	Y
7654	MARTIN	SALESMAN	7698	28-Sep-81	1250	1400	50	N
7698	BLAKE	MANAGER	7839	1-May-81	2850	(null)	30	Y
7782	CLARK	MANAGER	7839	9-Jun-81	2450	(null)	10	N
7788	SCOTT	ANALYST	7566	19-Apr-87	3000	(null)	20	Y
7839	KING	PRESIDENT	(null)	17-Nov-81	5000	(null)	10	N
7844	TURNER	SALESMAN	7698	8-Sep-81	1500	0	30	Y
7876	ADAMS	CLERK	7788	23-May-87	1100	(null)	20	N
7900	JAMES	CLERK	7698	3-Dec-81	950	(null)	30	Y
7902	FORD	ANALYST	7566	3-Dec-81	3000	(null)	20	N
7934	MILLER	CLERK	7782	23-Jan-82	1300	(null)	60	Y

If the below queries were fired, to get the list of departments that does not have any employee, one row (department no 70) should be returned.

SELECT deptno FROM dept WHERE deptno NOT IN (SELECT deptno FROM emp);

and

SELECT deptno FROM dept
MINUS
SELECT deptno FROM emp;

But surprisingly, first query will return no rows and the second will return the department no 70 because this dept does not have any employee.

Why? Its because, in the first query, each deptno from dept table will be compared with all the deptnos along with the NULL in the emp table. Remember, comparison of a value with NULL will be FALSE. So the deptno 70 comparison with NULL will be false and deptno 70 will not be returned.