sql 优化
https://www.cnblogs.com/williamjie/p/9389530.html
Mysql优化之GROUP BY语句优化
https://blog.csdn.net/qq_27037443/article/details/93876706
MySQL巧建sum索引帮我们提高至少100%的效率
https://www.cnblogs.com/sunss/archive/2010/10/30/1864949.html
讲真，MySQL索引优化看这篇文章就够了
https://cloud.tencent.com/developer/news/362898
高效MYSQL性能优化--小结
https://blog.csdn.net/iteye_19607/article/details/82485950
MySQL索引背后的数据结构及算法原理
http://blog.codinglabs.org/articles/theory-of-mysql-index.html
mysql 子查询 EXISTS
https://www.cnblogs.com/mryangbo/p/10904197.html