Database Design: Composite key vs one column primary key

Question

A web application I am working on has encountered an unexpected \'bug\' - The database of the app has two tables (among many others) called \'States\' and \'Cities\'.

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Answer 1

It sounds like you are gathering data for a telephone directory. Are you? Why are states important to you? The answer to this question will probably determine which database design will work best for you.

You may think that it's obvious what a city is. It's not. It depends on what you are going to do with the data. In the US, there is this unit called MSA (Metropolitan Statistical Area). The Kansas City MSA spans both Kansas City, Kansas and Kansas City, Missouri. Whether the MSA unit makes sense or not depends on the intended use of the data. If you used area codes in US to determine cities, you'd end up with a very different grouping than MSAs. Again, it depends on what you are going to do with the data.

In general whenever hierarchical patterns of political subdivisions break down, the most general solution is to consider the relationship many-to-many. You solve this problem the same way you solve other many-to-many problems. By creating a new table, with two foreign keys. In this case the foreign keys are IdAreacode and IdStates.

Now you can have one arecode in many states and one state spanning many area codes. It seems a shame to accpet this extra overhead to cover just one exception. Do you know whether the exception you have uncovered is just the tip of the iceberg, and there are many such exceptions?