SELECT SUM(s.quantity) - r.reserved
    FROM stock s
    INNER JOIN (
        SELECT COUNT(r.id_reservation) reserved
        FROM stock s
        LEFT JOIN reservation r ON r.fk_stock = s.id_stock
        WHERE s.simple_sku IN (SELECT csi.sku FROM catalog_simple csi WHERE csi.fk_catalog_config = id_catalog_config)
    ) r
    WHERE s.simple_sku IN (SELECT csi.sku FROM catalog_simple csi WHERE csi.fk_catalog_config = id_catalog_config)