SELECT nameCustomer AS customer_name, (
    SELECT idItem
      FROM detailSale
      INNER JOIN sales on sales.idDetail = detailSale.idDetail
      WHERE sales.idCustomer = cliente.idCustomer
      GROUP BY idItem
      ORDER BY SUM(amountBuyItem) DESC
      LIMIT 1
  )
  FROM cliente

SELECT idCustomer, idItem, SUM(amountBuyItem) AS totalBuyItem
  FROM detailSale
  INNER JOIN sales on sales.idDetail = detailSale.idDetail
  GROUP BY idCustomer, idItem
  ORDER BY idCustomer, idItem DESC