Database Structure Advice Needed

Question

Im currently working on a site which will contain a products catalog. I am a little new to database design so I\'m looking for advice on how best to do this. I am familiar wit

Answer 1

Before you create a hierarchical category model in your database, take a look at this article which explains the problems and the solution (using nested sets).

To summarize, using a simple parent_category_id doesn't scale very well and you'll have a hard time writing performant SQL queries. The answer is to use nested sets which make you visualize your many-to-many category model as sets which are nested inside other sets.

It should be worth pointing out that the "multiple categories" idea is basically how "tagging" works. With the exception that, in "tagging", we allow any product to have many categories. By allowing any product to be in many categories, you allow the customer the full ability to filter their search by starting where they believe they need to start. It could be clicking on "airplanes", then "wood", then "turbojet engine" (or whatever). Or they could start their search with Wood, and get the same result.

This will give you the greatest flexibility, and the customer will enjoy a better UX, yet still allow you to maintain the hierarchy structure. So, while the quoted answer suggests letting categories be M:N to categories, my suggestion is to allow products to have M:N categories instead.

All in all the result is mostly the same, the categories will have a natural hierarchy, but this will lend to even greater flexibility.

I should also note that this doesn't prevent strict hierarchy either. You could much easily enforce hierarchy in the code where necessary (ex. only showing the categories "cars", "airplanes", and "boats" on your initial page). It just moves the "strctness" to your business logic, which might make it better in the long run.

EDIT: I just realized that you vaguly mentioned this in your answer. I actually didn't notice it, but I think this is along the lines you would want to do instead. Otherwise you are mixing two hierarchy systems into your program without much benefit.