Couldnt find anything online so ill let you know what i do...
I have a table for all the users info... So it has user,pass,a unique ID number, firstname,lastname...etc...
So table structure would look like this:
| user | pass | u_id | first | last | email_addr |
I then make another table to store what ever info is going to be associated with the users... Lets say its going to store comments the user has submitted.
The table would look something like this:
| c_id | u_id | comment_info | date |
When the users submits a comment I would make a unique ID for the comment and make sure to also insert the users unique ID (u_id) along with the rest of the info.
To grab only that users comments I would make a query like this:
$query = "SELECT * FROM comments WHERE u_id = '{$users_id}'";
This would only take the comments where the users id matches the u_id in the table.
Its all about organization from with in your database.