Calculate the quantity percentage of the gift products in the total trading quantity.
percentage = MULTIPLY(DIVIDE(SUM(Quantity WHERE Price = 0), SUM(Quantity)), 1.0); gift products refers to Price = 0;
SELECT CAST(SUM(IIF(T1.Price = 0, T2.Quantity, 0)) AS REAL) * 100 / SUM(T2.Quantity)FROM Products AS T1 INNER JOIN Sales AS T2 ON T1.ProductID = T2.ProductID