Another thing that is worth metioning in this "drop down list" thing is when, for example, you want to create a sales report based on the user selection of a product.
Let's imagin that your parameters have a StartDate, an EndDate and a Product List (drop down). The end user will be prompt to choose the value for the 3 parameters and will then receive the according report. But what if the user wants to see more than one product but not all of them? For these scenarios there's a parameter property called multi-value, which allows you to select all, some, or none, of the available items in the drop down list. To make your query work in this scenario you will have to change it to something like this
Code:
SELECT
<something>
FROM
Sales.SalesLog sl
WHERE
sl.transactionDate BETWEEN @startDate AND @endDate
AND sl.productId IN (@productId)
Best regards and happy programming in SQL Server Reporting Services.