|A foreign key is a primary key from another table. The "username" in the Users table (calling it userstable is redundant, all those things are tables and not chairs with little chance of confusing those) is not a foreign key - but it might be candidate-primary key.
Also, the artificial autonumber may be unique, but it does not mean that one can assign it blindly to be the primary key without asking oneself what the user will be using to uniquely identify a record.
You should look into normalization.
Bastard Programmer from Hell
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