SELECT 
    JSON_OBJECT
    (
         'departmentId' is d.DEPARTMENT_ID,
         'name' is d.DEPARTMENT_NAME,
         'employees' is 
         (
             SELECT JSON_ARRAYAGG
             (
                 JSON_OBJECT
                 (
                     'employeeId' is E.EMPLOYEE_ID,
                     'firstName' is E.FIRST_NAME,
                     'lastName' is E.LAST_NAME,
                     'emailAddress' is E.EMAIL
                  )
              )
              FROM HR.EMPLOYEES E
              WHERE E.DEPARTMENT_ID = D.DEPARTMENT_ID
          )
      ) DEPT_WITH_EMPLOYEES
FROM HR.DEPARTMENTS D
WHERE D.DEPARTMENT_ID = 100 ;