What's better - many small tables or one big table?

Question

I've got a database which will store profiles about individuals. These individuals have about 50 possible fields.

Some are common things like, first name, last name, email, phone number.

Others are things like hobbies, skills, interests

Some are height, weight, skin color.

Each of these groups are used by the system at different times. In terms of being able to negotiate through the database I would prefer to have 7 tables each of about 8 fields. What is best practice to do?

EDIT: The data is going to be used in a search engine, for finding profile matches. Does this affect what I am doing?

Answer 1

I'm with the Normalize camp.

Here are a few hints to get you started:

Start with a process to assign some arbitrary unique identifier to each "person". Call this the PersonId or something like that. This identifier is called a surrogate key. The sole purpose of a surrogate key is to guarantees a 1 to 1 relationship between it and a real person in the real world. Use the surrogate key when associating the value of some other attribute to a "person" in your database.

As you develop your database layout you may find surrogate keys necessary (or at least useful) for some other attributes as well.

Look at each attribute you want to manage. Ask the following question: Does any given person have only one value for this attribute?

For example, each person has exactly one "Birth Date". But how may "Hobbies" can they have? Probably zero to many. Single valued attributes (eg. Birth date, height, weight etc.) are candidates to go into a common table with PersonId as the key. The number of attributes in each table should not be of concern at this point.

Multi valued attributes such as Hobby need a slightly different treatment. You might want to create separate tables for each multi-valued attribute. Using Hobbies as an example you might create the following table PersonHobby(PersonId, Hobby). A row in this table might look something like: (123, "Stamp Collecting"). This way you can record as many hobbies as required for each person, one per row. Do the same for "Interest", "Skill" etc.

If there are quite a number of multi-valued attributes where the combination of PersonId + Hobby determine nothing else (ie. you don't have anything interesting to record about this person doing this "Hobby" or "Interest" or "Skill") you could lump them into an Attribute-Value table having a structure something like PersonAV(PersonId, AttributeName, Value). Here a row might look like: (123, "Hobby", "Stamp Collecting").

If you go this route, it is also a good idea to substitute the AttributeName in the PersonAV table for a surrogate key and create another table to relate this key to its description. Something like: Attribute(AttributeId, AttributeName). A row in this table would look something like (1, "Hobby") and a corresponding PersonAV row could be (123, 1, "Stamp Collecting"). This is commonly done so that if you ever need to know which AttributeNames are valid in your database/application you have a place to look them up. Think about how you might validate whether "Interest" is a valid value for AttributeName or not - if you haven’t recorded some person having that AttributeName then there is no record of that AttributeName on your database - how do you know if it should exist or not? Well look it up in the Attribute table!

Some attributes may have multiple relationships and that too will influence how tables are normalized. I didn't see any of these dependencies in your example so consider the following: Suppose we have a warehouse full of parts, the PartId determines its WeightClass, StockCount and ShipCost. This suggests a table something like: Part(PartId, WeightClass, StockCount, ShipCost). However if relationship exists between non-key attributes then they should be factored out. For example suppose WeightClass directly determines ShipCost. This implies that WeightClass alone is enough to determine ShipCost and ShipCost should be factored out of the Part table.

Normalization is a fairly subtle art. You need to identify the functional dependencies that exist between all of the attributes in your data model in order to do it properly. Just coming up with the functional dependencies takes a fair bit of thought and consideration - but it is critical to getting to the proper database design.

I encourage you to take the time to study normalization a bit more before building your database. A few days spent here will more than pay for itself down the road. Try doing some Google/Wikipedia searches for "Functional Dependency", "Normalization" and "Database Design". Read, study, learn, then build it right.

The suggestions I have made with respect to normalizing your database design are only a hint as to the direction you might need to take. Without having a strong grasp of all the data you are trying to manage in your application, any advice given here should be taken with a "grain of salt".

Answer 2

It is hard to say, and is based on what the application requires. I would say to look into Database Normalization as it will show you how to normalize the database and in that it should shed light on what you would want to separate out into their own tables etc.