Oracle 外连接实现代码

1. left outer join:左外关联
select e.last_name, e.department_id, d.department_name
from employees e
left outer join departments d
on (e.department_id = d.department_id);
等价于
select e.last_name, e.department_id, d.department_name
from employees e, departments d
where e.department_id=d.department_id(+);
结果为:所有员工及对应部门的记录，包括没有对应部门编号department_id的员工记录。
2. right outer join:右外关联
select e.last_name, e.department_id, d.department_name
from employees e
right outer join departments d
on (e.department_id = d.department_id);
等价于
select e.last_name, e.department_id, d.department_name
from employees e, departments d
where e.department_id(+)=d.department_id;
结果为:所有员工及对应部门的记录，包括没有任何员工的部门记录。
3. full outer join:全外关联
select e.last_name, e.department_id, d.department_name
from employees e
full outer join departments d
on (e.department_id = d.department_id);
结果为:所有员工及对应部门的记录，包括没有对应部门编号department_id的员工记录和没有任何员工的部门记录。