union:对多个结果集进行并集操作，不包括重复行，同时进行排序。
union all:对多个结果集进行并集操作，包括重复行，不进行排序。
查询部门小于30号的员工信息，和部门大于20小于40号的员工信息。
①.先查询部门小于30号的员工信息。
SELECT
employees_id
,last_name
,salary
,department_id
FROM employees
WHERE department_id < 30; ```
查询部门大于20小于40的员工信息。
SELECT
employees_id
,last_name
,salary
,department_id
FROM
employees
WHERE
department_id BETWEEN 20 and 40;```
③.用union连接两张表
SELECT
employees_id
,last_name
,salary
,department_id
FROM
employees
WHERE
department_id < 30
UNION
SELECT
employees_id
,last_name
,salary
,department_id
FROM
employees
WHERE
department_id BETWEEN 20 and 40; ```
其结果默认排序并去重，两张表都有30号部门信息，结果只出现一次。
④.下面用union all连接两张表
SELECT
employees_id
,last_name
,salary
,department_id
FROM
employees
WHERE
department_id < 30
UNION ALL
SELECT
employees_id
,last_name
,salary
,department_id
FROM
employees
WHERE
department_id BETWEEN 20 and 40; ```
其结果没有去重，也没有排序，排序结果对比下边结果，先去查询20到40的员工信息，在查小于30的员工信息。
⑤.对比查询结果
SELECT
employees_id
,last_name
,salary
,department_id
FROM
employees
WHERE
department_id BETWEEN 20 and 40
UNION ALL
SELECT
employees_id
,last_name
,salary
,department_id
FROM employees
WHERE department_id < 30;```
默认是没有进行排序的。
