1.oracle中联接(join)分为内联接(inner)和外联接(outer join)，而外联接(outer join)又分为 左外联接(left outer join)和右外联接(right outer join)。 2. select * from test2; select * from test3; (1)笛卡尔乘积(cartesian product) select * from test2
1.oracle中联接(join)分为内联接(inner)和外联接(outer join)，而外联接(outer join)又分为
左外联接(left outer join)和右外联接(right outer join)。
2.
select * from test2;
select * from test3;
(1)笛卡尔乘积(cartesian product)
select * from test2, test3;
结果:
(2)内联接(inner join)
select * from test2 inner join test3 on test3.c2 = test2.c2;
结果:
(3)外联接(outer join)--作用：由于显示悬浮元祖
  2.1 右外联接(right outer join)
select * from test2 right outer join test3 on test2.c2 = test3.c2;--usage 1 select * from test2, test3 where test2.c2(+) = test3.c2;--usage 2
上述2条语句均表示右外联接
结果:
2.2 左外联接(left right join)
select * from test2 left outer join test3 on test2.c2 = test3.c2;--usage 1 select * from test2, test3 where test2.c2 =test3.c2(+);--usage 2
上述2条语句均表示左外联接
结果:
附录:数据库关于联接的概念
1.悬浮元素(dangling tuple):在一个联接中，若一个元组不能和另一个关系中的任何一个元组配对的话，该元组就成为悬浮元祖。
2.(1)笛卡尔乘积(2)积(3)交叉乘积 均表示同一个意思。
3.自然联接(natural join)
  3.1.先做条件联接
  3.2.对2个相同属性合并