Saved Reports - How Did They Do That?

I recently viewed an application which included customizable savable reports. In the first step, the user selects from a very large (probably over a hundred) set of variables to include in the report (these would be the columns displayed in the report). In a second step, the user selects (from the same set of variables) which variables they want to set as criteria indicators (these would then be dropdowns and text boxes on the report that would filter the query results).

So, how do they do this? Lets take the AdventureWorks database as an example. Lets say the user can select the following variables to include in their report:

Product.Name
Product.ProductNumber
Product.Color
Product.Size
Product.ListPrice
WorkOrder.StartDate
WorkOrder.EndDate
WorkOrder.OrderQty
PurchaseOrderDetail.DueDate
PurchaseOrderDetail.UnitPrice
PurchaseOrderDetail.OrderQty
Vendor.Name
Etc…

How do you design this in the database? If you have a table that holds all of the possible variables, and a table that connects the user-selected variables to the user – how then do you construct the query that pulls it all together? For example, what if a user wants only these two variables in their report:

Product.Name
Vendor.Name

Here is the query:

select p.name as ProductName, v.name as VendorName
from Production.Product as p
inner join Purchasing.ProductVendor as pv on pv.ProductID = p.ProductId
inner join Purchasing.Vendor as v on v.VendorId = pv.VendorId

How do you build that query from just getting two variable names?