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It isn't, particularly. Suppose, however, that you had all the user information (name, address, etc.) in one table and the password and security question/answer in another table. The efficient link would be via key. Joe Blow has the key 10001 in the user table and Joe Blow's password record has the key 10001 in the passwords table.
You will be more efficient if you learn databases via tutorial material (books, etc.) than if you learn them by inspecting existing databases.
Start with relational databases and don't forget to check out normalization.
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