之前写错了， 这个才是我自己改的
-- 左外连接 取经四人组中不是悟空的兄弟的人
SELECT DISTINCT a.`id`,a.`user_name`
FROM user_a AS a LEFT OUTER JOIN user_b AS b ON a.`user_name`=a.`user_name`
WHERE a.`user_name` NOT IN (SELECT user_name FROM user_b);
执行结果是没错 意思也有的对上 但是总觉得有点繁琐。
恕我直言一开始就应该给个完整表结构，对照表，写sql语句验证效果才会更容易理解。大家只是希望可以更加易懂，我没有说老师讲得不好，只是有的细节需要注意一下。
因为不知道完整的表结构，我直接按照老师给的两张表来建表，之后进行左外连接的测试 发现结果是空，我就自己按理解改了一下SQL语句
-- 左外连接 取经四人组中不是悟空的兄弟的人
SELECT DISTINCT a.`user_name`
FROM user_a AS a LEFT OUTER JOIN user_b AS b ON a.`user_name`=a.`user_name`
WHERE b.`user_name` IS NULL;
2. 优化使用join 语句
select a.user_name , b.timestr , b.kills
from user1 a
join user_kills b on a.id = b.user_id
join user_kills c on c.user_id = b.user_id
group by a.user_name , b.timestr , b.kills
having b.kills = MAX(c.kills)
1. 使用子查询语句：
select a.user_name,b.timestr,b.kills from user1 as a left join user_kills as b on a.id = b.user_id
where b.kills = (select max(c.kills) from user_kills as c where b.user_id = c.user_id);
