在讲外连接之前，先举例介绍内连接，也就是一般的相等连接。
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);
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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(+);
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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);
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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;
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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);
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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。
