外连接：内连接结果+主表中有而从表没有的结果
左外连接：left join 左边的是主表
右外连接：right join 右边的是主表

#案例：查询男朋友不在男神表的女神名
#左连接

SELECT
    b.name
FROM
    beauty b
LEFT OUTER JOIN
    boys bo
ON
    b.boyfriend_id=bo.id
WHERE
    bo.`id` IS NULL;

#右连接

SELECT
    b.name
FROM
    boys bo
RIGHT OUTER JOIN
    beauty b
ON
    b.`boyfriend_id`=bo.`id`
WHERE
    bo.id IS NULL;

#案例:查询哪个部门没有员工

SELECT
    d.department_name
FROM
    employees e
RIGHT OUTER JOIN
    departments d
ON 
    e.`department_id`=d.`department_id`
WHERE
    e.`department_id` IS NULL;