数据库基础入门 — 关联查询

--多表查询 以及多表查询在from后面的别名命名规范（在select和where上都要使用，否则报错）
--内连接1.
select e.last_name,d.department_name,l.city
from employees e,departments d,locations l
where e.department_id = d.department_id and d.location_id = l.location_id
--SQL99实现内连接2.(inner可以省略）
select e.last_name,d.department_name,l.city
from employees e inner join departments d
on e.department_id = d.department_id
join locations l 
on d.location_id = l.location_id
--左外连接（加上left后outer也可以省略）
select e.last_name,d.department_name
from employees e left outer  join departments d
on e.department_id = d.department_id
--右外连接（加上left后outer也可以省略）
select e.last_name,d.department_name
from employees e right outer  join departments d
on e.department_id = d.department_id
/*满外连接
可通过union 或者 union all实现
union：去重复 union all:不去重
*/
select e.last_name,d.department_name
from employees e left outer  join departments d
on e.department_id = d.department_id
union all
select e.last_name,d.department_name
from employees e right outer  join departments d
on e.department_id = d.department_id