How to correctly query # of items fulfilled

Hi all -

I am trying to query the transactions and transaction_lines tables for item fulfillments; what I want is to report on, for each item, how many have been fulfilled over the past 365 days. However, the ODBC connection seems to return multiple lines per item - e.g. if qty of 2 items were fulfilled on a transaction, there could be a line for that item showing +2, then a line showing -2, then a line showing +2 again. However, unfortunately this is not regular and seems to be random - e.g. if you simply SUM all the line quantities grouped by item, some totals result in a positive amount, some in a negative