Merge text files using field matching

We have a couple of instances where we want to extract a particular view in a MS SQL Server data base (lets call this A) with older, similar data in a text file from a legacy system (B). The output should be in text file - csv (C). The goal is produce a single file C we can do analysis on. The starting point is always A - all the rows and all the fields. We then want to add all the rows in B, matching up the field according to a table. Example: A Account Invoice Product A0001 1233 abc B Customer Invoice Stock Code B002 2344 GHJ using the field mappings we define, either tin the code or another file A field B field Account = Customer Invoice = Invoice Product = Stock Code This would give as output C: Account Invoice Product A0001 1233 ab B002 2344 GHJ There will never be more fields in B than A buthere could be less (use blank or null if so). I think we'd prefer python but dont really mind. The field mappings and the input and output files should be edittable, but this can be in the code. We want to run the program in batch mode, no GUI etc needed - just something we can schedule to run over night. We have data now.