10-SET运算符

将多个查询用 SET 操作符连接组成一个新的查询
　　-UNION/UNION ALL
　　-INTERSECT
　　-MINUS
排序:ORDER BY

--UNION： 操作符返回两个查询的结果集的并集
SELECT employee_id, job_id
FROM   employees
UNION
SELECT employee_id, job_id
FROM   job_history;

--UNION ALL 操作符返回两个查询的结果集的并集。对于两个结果集的重复部分，不去重。
SELECT employee_id, job_id, department_id
FROM   employees
UNION ALL
SELECT employee_id, job_id, department_id
FROM   job_history
ORDER BY  employee_id;

--INTERSECT 操作符返回两个结果集的交集
SELECT employee_id, job_id
FROM   employees
INTERSECT
SELECT employee_id, job_id
FROM   job_history;

--MINUS操作符：返回两个结果集的差集
SELECT employee_id,job_id
FROM   employees
MINUS
SELECT employee_id,job_id
FROM   job_history;

 

使用 SET 操作符注意事项
*在SELECT 列表中的列名和表达式在数量和数据类型上要相对应
*括号可以改变执行的顺序
*ORDER BY 子句:
　　只能在语句的最后出现
　　可以使用第一个查询中的列名, 别名或相对位置

*除 UNION ALL之外，系统会自动将重复的记录删除
*系统将第一个查询的列名显示在输出中
*除 UNION ALL之外，系统自动按照第一个查询中的第一个列的升序排列

 

--匹配各SELECT 语句举例1
SELECT department_id, TO_NUMBER(null) 
       location, hire_date
FROM   employees
UNION
SELECT department_id, location_id,  TO_DATE(null)
FROM   departments;

--匹配各SELECT 语句举例2
SELECT employee_id, job_id,salary
FROM   employees
UNION
SELECT employee_id, job_id,0
FROM   job_history;

--使用相对位置排序举例
COLUMN a_dummy NOPRINT --查询结果不显示a_dummy列
SELECT 'sing' AS "My dream", 1 a_dummy
FROM dual
UNION
SELECT 'I`d like to teach', 2
FROM dual
UNION 
SELECT 'the world to',3
FROM dual
ORDER BY 2 asc;

 

eg：

--查询部门的部门号，其中不包括job_id是”ST_CLERK”的部门号
--select department_id
--from departments
--where department_id not in (
--                           select distinct department_id
--                           from employees
--                           where job_id = 'ST_CLERK'
--                           )
select department_id
from departments
minus
select department_id
from employees
where job_id = 'ST_CLERK';

--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
column a_dummy noprint;
SELECT job_id,department_id,1 a_dummy
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;

--查询所有员工的last_name ,department_id 和department_name
select last_name,department_id,to_char(null)
from employees
union
select to_char(null),department_id,department_name
from departments;