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;