参考文章：
mysql 如何优化left join
mysql 创建索引和删除索引
mysql 查看索引
查看索引
show index from table_name;
show keys from table_name;
新建索引
alter table table_name add index index_name(column_name);
删除索引
alter table table_name drop index index_name;
查看字符编码
show full columns from table_name;
show create table table_name;
