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;