> (I'm not sure if a file should be allowed to belong to multiple tags within the same hierarchy; my gut says no.) Gmail is a good example. Has very flexible t

> (I'm not sure if a file should be allowed to belong to multiple tags within the same hierarchy; my gut says no.) Gmail is a good example. Has very flexible tagging schema. Every tag exists in a hierarchy. Emails can have multiple tags. SQL is not good for hierarchies though. You need to use recursive CTEs or denormalize relations and things.