Syntaxs Failure

Hey Guys can you help me to find the failure. I'am a new developer and have much to learn.

if(_custInvoiceTrans.CustAccount == _custVendExternalItem.CustVendRelation)
            {
               rawData.C6 = select firstOnly externalItemId from _CustVendExternalItem join _custInvoiceTrans where _custInvoiceTrans.CustAccount == _CustVendExternalItem.CustVendRelation
               && _custInvoiceTrans.ItemId == _CustVendExternalItem.ItemId;
            }

 else {
                rawData.C6 = "";

       }