Where query url FIlter

What do I have to do to make a UrlParameter in the select dynamic.

Examples:
I call up the page and there is still no UrlParameter
http://www.same.page.com/user

now the select looks like this:

select * from User where userid = *

and all users are displayed.

now i click on a link with linkparameter
http://www.same.page.com/user?userid=10

now the select will look like this

select * from User where userid = 10

How can I do that