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, 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。