Help with Sub Query

SELECT
*

FROM (
      SELECT Tracking_,
              pa.Purchased_Receivable,
              MID(pa.Purchased_Receivable,2,15),
              a.Selling_Price,
              'Activations' as `table`,
              pa.Customer_Name,
              a.Sold_By,
              a.Invoice_,
              a.Invoiced_By,
              a.Sold_On
      FROM  dp_expected as a
      LEFT JOIN disc_Activations as pa
      ON a.Tracking_ = pa.Mobile_ID
      WHERE MID(a.Selling_Price ,2,15) != MID(pa.Purchased_Receivable,2,15)
       AND pa.Purchased_Receivable IS NOT NULL
       AND pa.Purchased_Receivable != ''
       AND a.Selling_Price LIKE '%-%'
       AND STR_TO_DATE(a.Sold_On, '%b %e, %Y %l:%i %p')  BETWEEN '".$mindate."' AND '".$maxdate."'
      GROUP BY a.Tracking_

      UNION ALL

      SELECT Tracking_,
              pa.Purchased_Receivable,
              MID(pa.Purchased_Receivable,2,15),
              a.Selling_Price,
              'Upgrades' as `table`,
              pa.Customer_Name,
              a.Sold_By,
              a.Invoice_,
              a.Invoiced_By,
              a.Sold_On,
              a.ID

      FROM  dp_expected as a
      LEFT JOIN disc_Upgrades as pa
      ON a.Tracking_ = pa.Mobile_ID
       WHERE MID(a.Selling_Price ,2,15) != MID(pa.Purchased_Receivable,2,15)
        AND pa.Purchased_Receivable IS NOT NULL
        AND pa.Purchased_Receivable != ''
        AND a.Selling_Price LIKE '%-%'
        AND STR_TO_DATE(a.Sold_On, '%b %e, %Y %l:%i %p')  BETWEEN '".$mindate."' AND '".$maxdate."'
      GROUP BY a.Tracking_

) as x

;