Add aggregated column into the same table

I have a table in power query with the following columns:

• Sales person
• Date of sale
• Amount

I want to add another column which would sum monthly sales for each sales person. So this column would have many duplicated numbers.

I tried to created a new grouped table, and then to merge it with original table, but the power query didn't like the cyclic reference. Is there a way I have aggregated amount in the same table?

The table is constantly updated, so the monthly amount will be recalculated every time. Then I will use it the following way - if mothly sales of a person reach a certain level, then the day's amount is going to be processed a certain way.

You need a proper date column but the in absence of that, here is an easy way in PQ.

Start:

Add column, extract, first 3

Group by, sum and all:

Result

Then expand: