column|employee_id|not_nullable|number(6)
column|first_name|nullable|varchar2(20)
column|last_name|not_nullable|varchar2(25)
column|email|not_nullable|varchar2(25)
column|phone_number|nullable|varchar2(20)
column|hire_date|not_nullable|date
column|job_id|not_nullable|varchar2(10)
column|salary|nullable|number(8,2)
column|commission_pct|nullable|number(2,2)
column|manager_id|nullable|number(6)
column|department_id|nullable|number(4)
check|(salary > 0) 
unique|email
pkey|employee_id
fkey|department_id|hr.departments.department_id
fkey|job_id|hr.jobs.job_id
fkey|manager_id|hr.employees.employee_id