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What does the "length" of a primary key refer to?

ariestav asked
Last Modified: 2012-05-12
This is just a simple question for which I simply don't understand conceptually.  Let's say I have a user table in my db, and I want to make a Primary Key based off the UserID field.  I know how to make the index and how to ensure it is unique, but what I never understood is what "Key Length" referred to.  Shouldn't the length of the primary key match that of the field from which is was created?  

Any light that you can shed on the topic of key length (particularly for MySQL) is greatly appreciated.

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