Elmah: to find more, query AllXml as Xml!
.value('(//queryString/item[@name="id"]/value/@string)','nvarchar(255)') as product_id
.exist('//item[@name="PATH_INFO"]/value[@string="/products/product.aspx"]') = 1
Since Elmah stores in the AllXml column many informations for the logged errors, those can be queried in the XPath/XQuery fashion. The AllXml values needs to be converted to the Xml type and the great thing is MS-SQL Server supports XML queries.
This sample outputs all values used for the "id" query string field, at the "/products/products.aspx" requests. In this way will get a good idea what to fix on that page.