SELECT bo.* FROM boys bo RIGHT OUTER JOIN beauty b ON b.`boyfriend_id`=bo.`id` WHERE b.`id`>3; #案例：查詢哪個(gè)城市沒(méi)有部門(mén)
SELECT city FROM locations l LEFT OUTER JOIN departments d ON l.`location_id`=d.`location_id` WHERE d.`department_id` IS NULL; #案例:查詢部門(mén)名為SAL或IT的員工信息（用外連接，內(nèi)連接有可能沒(méi)有員工，查詢不出來(lái)） #用外連接，因?yàn)榭赡苡械牟块T(mén)名為上述，但沒(méi)有員工，以null來(lái)填充員工表 #用內(nèi)連接，只會(huì)匹配有員工的SAL或IT，會(huì)忽視掉外連接中主表有但從表沒(méi)有的
SELECT e.*,d.`department_name` FROM departments d LEFT JOIN employees e ON d.`department_id`=e.`department_id` WHERE d.`department_name` IN ('SAL','IT');
