Compute keys that appear only on the first list; for example, a customer on the master who is absent from this period.
Subtract two Excel lists on an id and keep keys that are in A and missing from B.
Upload two Excel workbooks with an id column and subtract them on that exact id.
Download the template pack for a filled sample workbook and a header-only blank workbook. Map the required columns, compute in the browser, then export a result sheet. No signup.