Hello,
thanks for reading! 
Ich will describe my problem with the help of
an example, I think, that's the quickiest way to present my thoughts.
Let's assume, you want to save locations. Locations can be of different types: country, federal state, town or even a shop or a phone cell. These ways of modelling this came to my mind:
1. Create a table for each type of location, identify the common properties and extract these to seperate tables, so that you could end up in something like this:
Tables: 'countries', 'federal_states', 'towns' and as an example for a table holding common properties 'demographic_information' which will be joined with each of the mentioned tables.
2. Identify location categories, whose members have the same properties and add a kind of type column referring to a location type table, so:
Tables: 'cat1_locations' with a foreign key to a 'cat1_location_types' table.
3. I could imagine to modell this polymorphic, but I've got no clear idea right now, maybe you have heard more about this and are willing to tell me more about this, I would really appreciate this.
Let's come to my question: Which way would you modell this? What other possibilities are there to modell this, maybe a kind of modelling pattern? Can you name me criterias on which my decision should be based?
Thank you really much!
Best wishes, ms