理解Oracle的各种连接方法的最有效的方法就是“躬亲”，在实践中去深刻理解内连接，左外连接，右外连接，全外连接的概念的和效果。
1.创建测试表并准备测试数据
sec@ora10g> create table a (a number(1),b number(1),c number(1));
sec@ora10g> create table b (a number(1),d number(1),e number(1));
sec@ora10g> insert into a values(1,1,1);
sec@ora10g> insert into a values(2,2,2);
sec@ora10g> insert into a values(3,3,3);
sec@ora10g> insert into b values(1,4,4);
sec@ora10g> insert into b values(2,5,5);
sec@ora10g> insert into b values(4,6,6);
sec@ora10g> commit;
sec@ora10g> select * from a;
A B C
---------- ---------- ----------
1 1 1
2 2 2
3 3 3
sec@ora10g> select * from b;
A D E
---------- ---------- ----------
1 4 4
2 5 5
4 6 6
2. 内连接
sec@ora10g> select * from a, b where a.a=b.a;
另外一种写法如下
sec@ora10g> select * from a inner join b on a.a=b.a;
A B C A D E
---------- ---------- ---------- ---------- ---------- ----------
1 1 1 1 4 4
2 2 2 2 5 5
3.左外连接
sec@ora10g> select * from a,b where a.a=b.a(+);
另外一种写法如下
sec@ora10g> select * from a left outer join b on a.a=b.a;
A B C A D E
---------- ---------- ---------- ---------- ---------- ----------
1 1 1 1 4 4
2 2 2 2 5 5
3 3 3
4.右外连接
sec@ora10g> select * from a,b where a.a(+)=b.a;
另外一种写法如下
sec@ora10g> select * from a right outer join b on a.a=b.a;
A B C A D E
---------- ---------- ---------- ---------- ---------- ----------
1 1 1 1 4 4
2 2 2 2 5 5
4 6 6
5.全外连接
sec@ora10g> select * from a full outer join b on a.a=b.a;
A B C A D E
---------- ---------- ---------- ---------- ---------- ----------
1 1 1 1 4 4
2 2 2 2 5 5
3 3 3
4 6 6
6.小结
通过使用Oracle提供的各种SQL连接功能可以解决诸多的“疑难杂症”，需灵活掌握。