equals与==,hashcode的区别
https://www.jianshu.com/p/b196d74f8aeb
https://www.cnblogs.com/bianfuxia/p/12064720.html
get与post的区别
https://www.iteye.com/blog/tctl-795859
mysql的find_in_set()函数的使用
https://www.cnblogs.com/xiaoxi/p/5889486.html
mysql的expain关键字的作用