$query = "SELECT mqe.registration,
                    COUNT(*) AS numberofenqs,
                    COUNT(DISTINCT ucv.ip) AS unique_views,
                    SUM(ucv.views) AS total_views
                    FROM main_quick_enquiries AS mqe
                    LEFT OUTER JOIN used_car_views AS ucv
                    ON ucv.numberplate = mqe.registration
                    WHERE mqe.registration IS NOT NULL
                    GROUP BY mqe.registration ORDER BY numberofenqs DESC";

SELECT registration, COUNT(*) AS numberofenqs FROM main_quick_enquiries GROUP BY registration ORDER BY numberofenqs DESC