Relating data from two different periods

Hi everyone,

I am starting to use the relational and Power Pivot databases, so I do not have much knowledge.

I have to make a comparative report between two different annual periods.

The purpose of the report is to compare the [Gross Margin] of 2017 with the Accumulated [Gross Margin] of 2018, which can be for a specific month or for the accumulated of several selected months, to see the difference between them.

I have two databases; one with total data for the year 2017 and another with disaggregated data for the months of 2018. In both databases [Gross Margin] is collected by [Customer], [Sales document], [Reporting Zone] and [Local Reporting Code].

How can I relate the 2017 data with the 2018 data? I have tried to create a code by concatenating the numbers of [Customer], [Sales document], [Reporting Zone] and [Local Reporting Code], in order to relate one year with another. The problem is that by 2018 that code does not work for me, since there can be two lines that are the same because there are data for several months.

I am blocked, since I do not know how to continue and I would be grateful if someone could shed some light on this issue or lend me their help.

Regards,

Lamati