Query for sales revenues in separate accounts?

Hi forumers !

Here is my problem:
I would like to distinguish sales revenus for an item depending on whether the transaction is taxable or not taxable. To this I created a UDF "CNTAX" in the item master data (Items-warehouse). CNTAX contains revenue accounts for non taxable operations. I also created a UDF "TYOP" at the header level in the A/R invoice form. TYOP records the operation type (Tax liable="Yes", not taxable="No").
I want that when the user selects the type of transaction (i.e "NO" for non-taxable), the revenu account displayed at row level for an item in an AR invoice is retrieved from "CNTAX".
Can someone send me the query to achieve this?