oracle中的左右连接

在oracle中左右连接省去了sql server中复杂的连接语句(left join ,right join)，就用一个"(+)"表示。
下面做了下实验：其中查询1和查询2是等同效果的，查询3和查询4是等同效果的。

查询1:
select first_name,department_name,emp.department_id from emp,departments dept
where emp.department_id(+)=dept.department_id;

122 rows selected.

查询2:
select first_name,department_name,emp.department_id from departments dept left join emp
on emp.department_id=dept.department_id;

122 rows selected.

 

 

查询3:
select first_name,department_name,emp.department_id from emp,departments dept
where emp.department_id=dept.department_id(+);

107 rows selected.

查询4:
select first_name,department_name,emp.department_id from  emp  left join departments dept
on emp.department_id=dept.department_id;

107 rows selected.

 

总结：（+）在哪一边，则返回另一边所有的记录。