Validation helper for login form?

[eluser]inktri[/eluser]
So there are input fields: Username and Password
I want to make a rule so that size of:

Code:

select id from users where username = 'username' and password = md5('password') limit 1;

must equal 1

This rule needs data in both Username and Password fields. How would you go about creating a rule which requires data from multiple fields such as the case above

Thanks for the help