column|employee_id|not_nullable|number(6)
column|start_date|not_nullable|date
column|end_date|not_nullable|date
column|job_id|not_nullable|varchar2(10)
column|department_id|nullable|number(4)
check|(end_date > start_date) 
pkey|employee_id, start_date
fkey|job_id|hr.jobs.job_id
fkey|employee_id|hr.employees.employee_id
fkey|department_id|hr.departments.department_id