SQL Script to find Contract Purchase Agreement Number R11i

I have written the following script to link ap invoices to po's and want to get the contract purchase agreement number we use in Oracle. I can get the contract_id but the contract_num from po lines is blank (yet when I log into Oracle I can see the cpa number).

My sql script is below. Can someone help me get the contract number ie G.CONTRACT_NUM in script below ?

select d.segment1 as vendor_num
    ,d.vendor_name as vendor
    ,c.invoice_num
    ,c.invoice_type_lookup_code as type  
    ,c.DESCRIPTION as invoice_desc
    ,c.invoice_date as invoice_dt