(1)查询男朋友不在男生表的女生名----------左外连接
SELECT b.name FROM beauty b LEFT OUTER JOIN boys bo ON b.`boyfriend_id`=bo.`id` WHERE bo.`id` IS NULL;
(2)查询男朋友不在男生表的女生名----------右外连接
SELECT b.name FROM boys bo RIGHT OUTER JOIN beauty b ON b.`boyfriend_id`=bo.`id` WHERE bo.`id` IS NULL;
(3)查询哪个部门没有员工-----------------左外连接
SELECT d.*, e.employee_id FROM `departments` d LEFT OUTER JOIN `employees` e ON d.`department_id`=e.`department_id` WHERE e.`department_id` IS NULL;
(4)查询哪个部门没有员工-----------------右外连接
SELECT d.*, e.employee_id FROM `employees` e RIGHT OUTER JOIN `departments` d ON d.`department_id`=e.`department_id` WHERE e.`department_id` IS NULL;