Query for assigning roles (repost)

If I understand your question correctly, your table names are confusing, I think this is partially why you haven't had a response. If you can re-name Roles to UserGroups, or better yet rename Roles to UserRoles and Groups to Roles then this will improve the understandibility of your tables, it current looks like you want to be able to assign users to both groups and roles (which is something a lot of people actually want to do).

Before you make the changes, the SQL will be something like:

INSERT INTO 
  ROLES (UserId,GroupId)
SELECT 
   UserId,
   GroupId
FROM 
  Users,
  Groups
WHERE
  UserName= 'Ali'
AND
  GroupName='Administrator'

You should parameterise this query to avoid SQL injection[^] so 'Ali' and 'Administrators' become parameters.

As for rolling it up into one table: don't. As Christain said in his [deleted] reply it will result in you only being able to assign one role to each user, this is almost certainly not what is needed as users normally have multiple roles.