I have 2 excel tables files.One table has info about sales by country, by model, by date...and the other table has units by date.

Obviously, the common key is "date"....and by creating a relationship, I can add "units" to my combined resulting table.

However, I can't create the relationship. Excel keeps telling me: "the relationship cannot be created because each column contains duplicate values. Select at least one column that contains only unique values".

Can anybody explain how to resolve this in a step-by-step way, since I am not good with excel.
Thanks.