--1
SELECT
    TO_CHAR(MAX(SALARY),'$999,999'),
    TO_CHAR(ROUND(AVG(SALARY)),'$999,999'),
    TO_CHAR(MIN(SALARY),'$999,999'),
    TO_CHAR(SUM(SALARY),'$999,999'),
    COUNT(EMPLOYEE_ID)
FROM EMPLOYEES;

--2
SELECT
    D.DEPARTMENT_NAME,
    COUNT(E.DEPARTMENT_ID)
FROM DEPARTMENTS D
JOIN EMPLOYEES E
ON E.DEPARTMENT_ID = D.DEPARTMENT_ID
WHERE ROWNUM = 1
group by D.DEPARTMENT_NAME, DEPARTMENT_NAME
HAVING COUNT(E.DEPARTMENT_ID) = (SELECT
                                    MIN(COUNT(E.DEPARTMENT_ID))
                                FROM DEPARTMENTS D
                                JOIN EMPLOYEES E
                                ON E.DEPARTMENT_ID = D.DEPARTMENT_ID
                                group by D.DEPARTMENT_NAME);

SELECT
    D.DEPARTMENT_NAME,
    COUNT(E.DEPARTMENT_ID)
FROM DEPARTMENTS D
JOIN EMPLOYEES E
ON E.DEPARTMENT_ID = D.DEPARTMENT_ID
group by D.DEPARTMENT_NAME, DEPARTMENT_NAME
HAVING COUNT(E.DEPARTMENT_ID) = (SELECT
                                    MAX(COUNT(E.DEPARTMENT_ID))
                                FROM DEPARTMENTS D
                                JOIN EMPLOYEES E
                                ON E.DEPARTMENT_ID = D.DEPARTMENT_ID
                                group by D.DEPARTMENT_NAME);


--3

SELECT
    FIRST_NAME || ' ' || LAST_NAME,
    TRUNC((SYSDATE - HIRE_DATE)/365),
    TO_CHAR(SALARY,'$999,999'),
    TO_CHAR(SALARY*((TRUNC((SYSDATE - HIRE_DATE)/365))/100),'$999,999')
FROM EMPLOYEES
WHERE TRUNC((SYSDATE - HIRE_DATE)/365) = (SELECT MAX(TRUNC((SYSDATE - HIRE_DATE)/365))
                                          FROM EMPLOYEES);

--4

SELECT
    MANAGER_ID,
    COUNT(EMPLOYEE_ID),
    COUNT(EMPLOYEE_ID)*1000

FROM EMPLOYEES
GROUP BY MANAGER_ID
HAVING COUNT(EMPLOYEE_ID) = (SELECT MAX(COUNT(MANAGER_ID))
                             FROM EMPLOYEES
                             GROUP BY MANAGER_ID);

SELECT
    FIRST_NAME || ' ' || LAST_NAME
FROM EMPLOYEES
WHERE EMPLOYEE_ID = (SELECT MANAGER_ID
                     FROM EMPLOYEES
                     GROUP BY MANAGER_ID
                     HAVING COUNT(EMPLOYEE_ID) = (SELECT MAX(COUNT(MANAGER_ID))
                                                  FROM EMPLOYEES
                                                  GROUP BY MANAGER_ID));