[b]1. LEFT OUTER JOIN:左外关联[/b] 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。 [b]2. RIGHT OUTER JOIN:右外关联[/b] 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。 [b]3. FULL OUTER JOIN:全外关联 [/b]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。