The question is not really about hierarchies, but more about representing diverse-but-similar domain entities. It's a problem found in GUI modeling also, where

The question is not really about hierarchies, but more about representing diverse-but-similar domain entities. It's a problem found in GUI modeling also, where various GUI widgets will or can have different properties. Some properties will be shared, such as "parent_ID" (AKA "container"), but many are specific to a particular widget or subset of widgets. Further, new widgets may be added down the road, and we don't want to have to invent a new table for each one. Current RDBMS modelling techniques handle these situations poorly. I've been a fan of "dynamic relational" to handle these kinds of problems, but so far it hasn't been implemented: https://news.ycombinator.com/item?id=15352786 https://news.ycombinator.com/item?id=15352786