彻底搞懂Oracle的左外连接和右外连接

Employees表:

    

 

Department表:

 

   

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);

 

select e.last_name, d.department_name

from employees e, departments d

where e.department_id = d.department_id(+)               左外连接

 

 

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。

 

 

 

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);

 

 

select e.last_name, d.department_id,d.department_name

from employees e, departments d

where e.department_id(+) = d.department_id;                  右外连接

 

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。

 

 

 总结:

A 表right outer join B表

结果是：所有B表记录，及匹配的A表记录,A表未匹配上的显示null

A(+)=B

A 表 left outer join B

A=B(+)

结果是：所有A表记录，及匹配的B表记录，B表未匹配的显示null