select employee_id, job_id from employees
union all
select employee_id, job_id from job_history;
select employee_id, job_id from employees
union
select employee_id, job_id from job_history; 没有重复值 触发排序
select employee_id, job_id from employees
intersect
select employee_id, job_id from job_history;
select employee_id from employees
minus
select employee_id from job_history;
select employee_id, job_id, salary from employees
union all
select employee_id, job_id, null from job_history;
select employee_id, job_id, to_char(salary) from employees
union all
select employee_id, job_id, 'no salary' from job_history;
集合排序：
select employee_id, job_id, salary from employees
union all
select employee_id, job_id, null from job_history
order by salary;
select employee_id, job_id, null from job_history
union all
select employee_id, job_id, salary from employees
order by 3;