one to one relationship This is my database structure:

Users {user_id, username}

Profiles {profile_id, user_id, user_views}

My question is: If I do it the way mentioned above, should I to set user_id as primary key or unique key? and what about profile_id?
The right way is like the following:

Users {user_id (PK), username (Unique)}

Profiles {profile_id(PK), user_id FK, user_views}


in the given structure, we are making user_id as PK to link between tables, and making the username as Unique to avoid duplication, some people will do the username as PK, but this will make a problem if you want to update the username in any related detailed table linked to that username, in general, you should make sure that your table has only the correct data, without duplication, easy to update, normalized and easy to select with the given PKs or FKs
...

Please read more info from the following reference:
http://dev.mysql.com/doc/refman/5.0/en/create-table-foreign-keys.html