As a side note: I would update the example at the beginning. The fact that you can change your twitter handle (even if it has to be unique) is a sign that it is a bad candidate for a primary key. Imagine that you use it as a primary key, and then the user changes it. It would require a major update all over the database. It would be better to have a different field as primary key, and have the twitter handle with an unique constraint.
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Nice introduction to primary keys.
As a side note: I would update the example at the beginning. The fact that you can change your twitter handle (even if it has to be unique) is a sign that it is a bad candidate for a primary key. Imagine that you use it as a primary key, and then the user changes it. It would require a major update all over the database. It would be better to have a different field as primary key, and have the twitter handle with an unique constraint.