oracle各種連結介绍
SQL> select * from a1;
BBB
----------
101
102
103
104
105

SQL> select * from a2;
BBB CCC
---------- --------------------
101
102
105

SQL> select * from a1,a2 where a1.bbb(+)=a2.bbb; 右連結

BBB BBB CCC
---------- ---------- --------------------
101   101
102   102
105   105

SQL> select * from a1,a2 where a1.bbb=a2.bbb(+); 左連結

BBB BBB CCC
---------- ---------- --------------------
101   101
102   102
103
104
105   105

------------------------------------------------------------
 select * from a, b where a.id = b.id;

对于外连接，Oracle中可以使用“(+)”来表示，9i可以使用LEFT/RIGHT/FULL OUTER JOIN，下面将配合实例一一介绍。

1. LEFT OUTER JOIN：左外关联
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);

等价于

SELECT e.last_name, e.department_id, d.department_name
FROM employees e, departments d
WHERE e.department_id=d.department_id(+);


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。

2. RIGHT OUTER JOIN：右外关联

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);

等价于

SELECT e.last_name, e.department_id, d.department_name
FROM employees e, departments d
WHERE e.department_id(+)=d.department_id;

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。

3. FULL OUTER JOIN：全外关联

SELECT e.last_name, e.department_id, d.department_name
FROM employees e
FULL OUTER JOIN departments d
ON (e.department_id = d.department_id);

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的员工记录和没有任何员工的部门记录。