Surpporting subqueries in IoTDB

The community plans to support subquery in the future. We need your help to figure out what kind of subquery is helpful in production environment. You can leave a comment here with an example sql. Detailed explaination can be very helpful.
Here is a draft document of subquery: [subquery](https://cwiki.apache.org/confluence/pages/viewpage.action?pageId=199529958)

先阅读链接的 Apache Confluence 子查询草稿，然后确定对生产环境有用的案例，并将示例 SQL 作为评论写在此 issue 中。完成的标准是：项目已记录具体的子查询需求和示例；该 issue 未指定实现文件或测试。

由索引模型根据 Issue 内容生成。