Oracle 外连接实现代码

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,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,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,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,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。