SQL: Select records where value does not belong to a certain column

I want to select those Supervisors that are not supervising any employee e.g. Sup4

Note: All the supervisors are employee themselves so the are in Employee Column but as the supervisors are not supervised by any one so the corresponding Supervisors Column is null.

Table: EmpData

I know a better approach would be to create a separate table for both Employee and Supervisor but I am just curious if there is any approach using join that I am missing.

I have tried following but it returns 0 records.

Executed in Oracle Live SQL:

CREATE TABLE EmpData(
    PK number(38) GENERATED ALWAYS as IDENTITY(START with 1 INCREMENT by 1) PRIMARY KEY,
    Employee varchar2(100) NOT NULL,
    Supervisor varchar2(100),
    SupOrEmpFlag varchar2(100) NOT NULL
);


INSERT INTO EmpData (Employee , Supervisor, SupOrEmpFlag) 
SELECT 'EmpA', 'Sup1', 'e' FROM dual UNION ALL 
SELECT 'Sup1',   null, 's' FROM dual UNION ALL 
SELECT 'EmpB', 'Sup2', 'e' FROM dual UNION ALL 
SELECT 'Sup2',   null, 's' FROM dual UNION ALL 
SELECT 'EmpC', 'Sup3', 'e' FROM dual UNION ALL 
SELECT 'Sup3',   null, 's' FROM dual UNION ALL 
SELECT 'Sup4',   null, 's' FROM dual

SELECT *
  FROM EmpData sup
  JOIN EmpData emp
    on emp.Employee = sup.Supervisor
   and sup.SupOrEmpFlag = 's'
  JOIN EmpData nemp
    on nemp.Employee = emp.Employee
   and nemp.Employee <> emp.Employee

Solution 1:

One option would be determining through use of hierarchical query such as

 SELECT NVL(supervisor,employee) AS supervisor
   FROM EmpData e
CONNECT BY PRIOR employee = supervisor
  GROUP BY NVL(supervisor,employee)
 HAVING MAX(level) = 1 

or using a query having a conditional aggregation provided for HAVING clause such as

 SELECT NVL(supervisor,employee) AS supervisor
   FROM EmpData e
  GROUP BY NVL(supervisor,employee) 
 HAVING MAX(CASE WHEN suporempflag = 's' AND supervisor IS NULL THEN 0 ELSE 1 END) = 0 

Demo

Solution 2:

SELECT * 
FROM EmpData
WHERE Employee NOT IN 
(
    SELECT
    DISTINCT(Supervisor)FROM EmpData
    WHERE SupOrEmpFlag = 'e'
)
and SupOrEmpFlag = 's'