Data Modeling: ethnicities with parent-child relationship?

Question

I have a site with users that I want users to be able to identify their ethnicities. What's the best way to model this if there is only 1 level of hierarchy?

Solution 1 (single table):

Ethnicity
- Id
- Parent Id
- Name

Solution 2 (two tables):

Ethnicity Group
- Id
- Name

Ethnicity
- Id
- Ethnicity Group Id
- Name

I will be using this so that users can search for other users based on ethnicity. Which of the 2 approaches will work better for me? Is there another approach I have not considered? I'm using MySQL.

Answer 1

Well there is such a thing as an Ethnicity Group in the real world, so you do need two tables, not one. The real world has three levels (the top-most would be Race), but I understand that may not be necessary here. If you squash the three levels into two, you have to be careful, and lay them all out properly at the beginning. However, they will be vulnerable to people saying they want the real thing, and you may have to change it, or change the structure to fit more in ... much more work later).

If you do it correctly, as per real world, that problem is eliminated. Let me know if you want Race, and I will change the model.

The tables are far too small, and the keys are too meaningful, to add Id-iot columns to them; leave them as pure Relational keys, otherwise you will lose the power of the Relational engine. If you really want narrow keys, use a CHAR(2) EthnicityCode, rather than a NUMERIC(10,0) or a meaningless number.

Link to Ethnicity Data Model (plus the answer to your other question)

Link to IDEF1X Notation for those who are unfamiliar with the Relational Modelling Standard.

Answer 2

If there is nothing like an "ethnicity group" in the real world, I'd suggest you don't introduce one in your data model.

All the queries you can do with the second one you can also do with the first one, because you can just select FROM ethnicity AS e1 JOIN ethnicity AS es ON (e2.ethnicity_id = e1.parent_id) .

Answer 3

I don't want to be awkward, but what are you going to do with people of mixed descent? I think that the best that you can hope for is a simple single-level enumeration like the kind of thing you get on census forms (eg 'Black', 'White', 'Asian', 'Hispanic' etc). It's not ideal, but it allows people to fairly easily self-identify. Concepts like race and ethnicity are wooly enough without trying to create additional (largely meaningless) hierarchies on top of them, so my gut feeling is to keep it simple.