How to group by and SUM in each line

The main problem I see here is that your categories overlap in Table2 the start month and end month are the same between rows, so odds are this will go wrong since the same 1month applies to 2 rows of Table1, thus double-counting that value. The current query is causing issues too because you are joining the tables, which results in limiting results to where start month actually equals 1month, thus not looking at the whole range of dates.
I would:
1. Remove the join
2. make the 1month criteria: "Between [Table2].[start month] And [Table2].[end month]"

But I don't know if you have the luxury of adjusting your Table2 contents. As it stands now, I am not certain that it is possible to get the output you want

I'd also like to understand the expected results... 202102 CZ to me wouldn't show 5+3 but rather just 5

Is this an Access question?

Hi Marcinrd,

You are almost there.
From Table1 Query, please edit it as below steps:

1. Remove the join (still keep Table1 and Table2 but no need to link between them).
2. Criteria of [1month] (last field) , please change to -> Between Table2.[start month] And Table2.[end month] .
3. First field , 1month|Table1|Group By, please change to start month|Table2|Group By .

Then you should get the result as expected, except for 202104 because in data in Table2 is currently 202104 and 202104 (not 202103).

Note that using 'Between' will slow down the query if data are very huge.