How to merge two datasets with different time periods

Hi,

I have two datasets (one with monthly data and one with quarterly data) about two different variables but about the same companies (see attachment).

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I want to merge them to make one dataset with monthly data. For the quarterly data, I want to convert that to monthly by just taking the last available quarterly data. So for example, for December 98 I want to have the LV ratio of December 98 (which is the end of the quarter) together with the monthly return of that company for December 98. For January 99, it would be the LV ratio of December 98 (latest available quarter), but the monthly return for January 99 (see attachment).

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I want to tell Excel to identify each company by its isin code and give from each sheet the LV ratio and return for a given time and for the LV ratio --> if date < next quarter but > previous quarter, take previous quarter.

I hope it's clear enough