I am a total novice at Excel, and I wonder if there are any Excel mavens out there who can help me with what I think is a relatively simple problem.

I have two csv files. For purposes of this forum I'll say it's data about sales. Each file has the following fields:
The name (and other demographic data) of the customer
The name of the sales rep
The date of sales call

File A contains a list of ALL my potential customers during a certain time.
File B contains a list of all customers who have actually purchased something from me.

So...file B is a subset of file A.

What I want to do is figure out, within a certain date range (say, by month or quarter) what percentage of my sales calls resulted in a sale, and also be able to determine this by percentage for each of my sales reps.

What is the best way to do this? What I've thought of so far is to add a column to each file called "sale" and then fill in every entry in file "B" with "yes" in this column, while leaving the column empty in file "A". Then copy all the data in file "B" and paste into file "A", then sort by name, so that I'll then have a full list of customers, some of which will be "duplicates". How do I then clear out the records that are blank in the sales column if there is also a corresponding record that has "yes" in that column?

Or is that the wrong approach? Do I use database functions?

I'm totally clueless here. Any help will be appreciated. Thanks, Gil