Complex SQL to see roles from groups?

Hi,
First post.... I'm trying to map roles to LDAP group membership and hoped I could get some help with how to analyse this please?

A role consists of between 1 and x groups from a set of x groups.
A role can only have a particular group once.
A user may have one role pr more roles (or no role because the groups they have don't describe any role).

Simple sample data:

Role      | Group
Helpdesk  | Password Reset
Security  | Password Reset
Security  | Enable User
Reception | Enable User
Reception | Disable User

The data from the LDAP is of the form

User    | Group
SmithA  | Password Reset
SmithA  | Enable User
BloggsJ | Password Reset
MouseM  | Enable User
MouseM  | Disable User

From that you can calculate the users have the following roles

User    | Role
SmithA  | Security
BloggsJ | Helpdesk
MouseM  | Reception

I'm struggling with the SQL needed to build a list of User's Roles and have a horrible feeling I'm missing the obvious. I've tied myself in knots with joins and unions but they're going nowhere.

P.S. I've asked this question on two other forums and so far nobody has been able to (or has chosen not to) give a proper answer.
If the question's not clear please do say.

Thanks