如何在创建索引时选择列？

This appear to be a weird question. I know different types of indexes in sql server (clustered, non-clustered, unique, filtered, index with included column(s) ...etc) and I know how to create them. Also I know that the index depend on the query but what I don't know is who choose column when creating the index. For example, suppose a simple website that allow users to post text and images. The website has a simple two tables shown in the image :This appear to be a weird question. I know diff