| id | name | id | name |
| 1 | zhangsan | 1 | hanmeimei |
| 2 | lisi | 2 | wangwu |
| 3 | wangwu |
join:select * from A join B on A.name=B.name -----取交集
| id | name | id | name |
| 1 | wangwu | 2 | wangwu |
full/out join:select * from A full join B on A.name=B.name ----取并集
| id | name | id | name |
| 1 | zhangsan | null | null |
| 2 | lisi | null | null |
| 3 | wangwu | 2 | wangwu |
| null | null | 1 | hanmeimei |
left join:select * from A left join B on A.name=B.name
| id | name | id | name |
| 1 | zhangsan | null | null |
| 2 | lisi | null | null |
| 3 | wangwu | 2 | wangwu |
right join: select * from A right join B on B.name
| id | name | id | name |
| null | null | 1 | hanmeimei |
| 3 | wangwu | 2 | wangwu |
Union: 会对两个结果集进行并集操作，不包括重复行
union all:对两个结果集进行并集操作，包括重复行
employee_china表:
employee_usa表:
union：
select E_ID,E_Name FROM employees_china
union
select E_ID,E_Name FROM employees_usa
去掉:
union all:
select E_ID,E_Name FROM employees_china
union all
select E_ID,E_Name FROM employees_usa
不能去掉:
当存在不相同的字段时: （employees_china 增加一个字段 E_Hello）
必须进行补充
select E_ID,E_Name,E_Hello FROM employees_china
union all
select E_ID,E_Name,' ' as E_Hello FROM employees_usa
union vs join
union 要求多个结果集列数相同，列类型相同
join是列的联合
