Help JOINS ORACLE SQL query

What you need in this case is to make a SELF JOIN , that is, make the union of tables with the same table of EMPLOYEES to be able to find the information of the manager.

The first point to correct is to stop using implicit% JOIN and start using explicit JOIN , since the functionality of using JOIN implicit is considered deprecated since 1992.

The query would be as follows:

SELECT 
     E.FIRST_NAME || ' ' || E.LAST_NAME AS "NOMBRE COMPLETO DEL EMPLEADO"
    ,D.DEPARTMENT_ID
    ,D.DEPARTMENT_NAME
    ,D.MANAGER_ID
    ,EE.FIRST_NAME || ' ' || EE.LAST_NAME AS "NOMBRE COMPLETO DEL MANAGER"
FROM EMPLOYEES E
    INNER JOIN DEPARTMENTS D
        ON E.DEPARMENT_ID = D.DEPARMENT_ID
    INNER JOIN EMPLOYEES EE
        ON E.MANAGER_ID = E.EMPLOYEE_ID

Magic occurs in JOIN :

INNER JOIN EMPLOYEES EE
        ON E.MANAGER_ID = E.EMPLOYEE_ID

If you can notice, by matching the field MANAGER_ID with EMPLOYEE_ID you are saying that you are going or get the manager's data through your employee id, since the manager is likewise another employee.

Note: For more information about the differences between JOIN implicit% and JOIN explicit, this great response can clear up the picture.

Following the previous answer as a basis for me worked like this

SELECT E.FIRST_NAME || ' ' || E.LAST_NAME AS EMPLEADO, 
       D.DEPARTMENT_NAME AS DEPARTAMENTO, 
       EE.FIRST_NAME || ' ' || EE.LAST_NAME AS MANAGER 
       FROM EMPLOYEES E, 
            DEPARTMENTS D 
       JOIN EMPLOYEES EE 
            ON (D.MANAGER_ID=EE.EMPLOYEE_ID);