1.union/union all
1.必须具有对应的列数以及数据类型
2.默认按照第一列从小到大的顺序排列
3.union 去重且排序；
4.union all 不去重且不拍序。
--employees01 1-70,33-80;employees01 3-90,33-80 【employee01有70号部门1人，80号部门3人，一下类似】 --查找去重之后两表的并集-union-37 /* select employee_id,department_id from employees01 union select employee_id,department_id from employees02 */ --查找去重之前两表的并集-union-70 /* select employee_id,department_id from employees01 union all select employee_id,department_id from employees02 */
2.intersect
--取二者的交集-intersect-33-80 /* select employee_id,department_id from employees01 intersect select employee_id,department_id from employees02 */
3.minus
前表-二者共有的
--取二者的差集-minus-1-70 /* select employee_id,department_id from employees01 minus select employee_id,department_id from employees02 */
4.使用相对位置进行排序
--查询10,50，,20号部门的job_id,department_id 并且 department_id按10,50,20的顺序排列 /* select job_id,department_id,1 from employees where department_id = 10 union select job_id,department_id,2 from employees where department_id = 50 union select job_id,department_id,3 from employees where department_id = 20 order by 3 asc */ --column a_dummy noprin; 不打印该别名对应的列 --查询所有员工的last_name,department_id,department_name /* select last_name,department_id,to_char(null) from employees union select to_char(null),department_id,department_name from departments */
