Inheritance in Database Design

Question

I am designing a new laboratory database with MANY types of my main entities.

The table for each entity will hold fields common to ALL types of that entity (entity_i

Answer 1

I think it depends mostly on how you want to use this data.

First of all, I don't really see the benefit of option 3 over option 2. I think separating the special tables in another schema will make your application harder to maintain, especially if later on commonalities are found between 'special values'.

As another option I would say: - Store the special values in an XML fragment (or blob). Most databases have ability to query on XML structures these days, so without the need for many extra tables, you would keep your flexibility for a small performance hit.

If you put all the special values in one table, you get a very sparse table. Most normal DBMSes cannot handle this very well, but there are some implementations that specialize in this. You could benefit from that.

Do you often need to query the key-value pairs? if you basically access that table through it's entry_id, I think having a key-value table is not a bad design. An extra index on the kay column might even help you when you do need to query for special values. If you build an application layer on top of your database, the key-value table will map on a Map or Hash structure, which can also easily be used.

It also depends on the different types of values you want to store. If there are many different types, that need to be easily accessed (instead of being serialized/deserialized to XML/Character-String) you might want to store the type in a separate column, but that will usually lead to a very complicated design.

Hope this helps (a little bit).

-Maarten