Database File Merge and Processing Programme

I have a TAG file which has on average 4.7 million entries. I also have a .csv file which also has on average 4.7 million entries. Both files are lists of books so they share a common field (ISBN number). I need to merge these two files so that I can then export the data based upon the results of the 'In Stock' column. I would expect that there will be approximately 3 million entries in the list that have stock available. I need to get this into excel which would not handle 3 million entries. What I need the programme to do is create .csv excel files with no more than 50,000 entries for ease of handling. Therefore if there are 3 million items in stock the programme would create 60 excel files. This will be a monthly process so the programme that is created to do this needs to be robust and reliable. If you have any questions then please ask!