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)

Read the linked Apache Confluence subquery draft first, then identify production-useful cases and write example SQL as a comment on this issue. Done means the project has documented concrete subquery requirements and examples; the issue does not name implementation files or tests.

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