Write SQL query to solve given problem: What is the percentage, by number of sales order units, for orders with quantities not greater than 3 and a discount of 0.2?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT CAST(SUM(CASE WHEN OrderQty < 3 AND UnitPriceDiscount = 0.2 THEN 1 ELSE 0 END) AS REAL) / COUNT(SalesOrderID) FROM SalesOrderDetail