Try this:
SELECT aspnet_Users.UserName, aspnet_Users.UserId
FROM aspnet_Users INNER JOIN
aspnet_UsersInRoles ON aspnet_Users.UserId = aspnet_UsersInRoles.UserId INNER JOIN
aspnet_Roles ON aspnet_UsersInRoles.RoleId = aspnet_Roles.RoleId
WHERE (aspnet_Roles.RoleName = 'admin')
Hope it help