sql中exists,not exists的用法 - 飞翔-方向 积累 沉淀 - 博客园
http://www.cnblogs.com/mytechblog/articles/2105785.html
NOT IN vs. NOT EXISTS vs. LEFT JOIN / IS NULL: SQL Server at EXPLAIN EXTENDED
https://explainextended.com/2009/09/15/not-in-vs-not-exists-vs-left-join-is-null-sql-server/
在sql语句多表连接中，in、exists、join那个效率更高一点。_百度知道
https://zhidao.baidu.com/question/228314587.ht
out join - 国内版 Bing
https://cn.bing.com/search?FORM=U227DF&PC=U227&q=out+join
inner join和out join的区别 - 小贱 - 博客园
http://www.cnblogs.com/tangjian/archive/2009/02/20/1395001.html