LEFT join mfg_vendor_relationship on mfg_vendor_relationship.mfg_key = mfg_part.mfg_key AND mfg_vendor_relationship.project_key = bom_item.project_key LEFT join vendor on mfg_vendor_relationship.vendor_key = vendor.vendor_key
Perhaps I am missing something, but it seems like just a matter of changing inner join to left so you keep what you already have and augment vendor information when it exists. I never use right joins (I re-write to always declare as left) so I am not sure if the right join near the top screws that up. I might consider using a UNION ALL to combine sets where values are NULL with left joins, and use plain (inner) joins for the not nulls.