Sum based on date range

None of your start/end dates are between the valid from/to dates, so nothing will be returned.

I added some rows where it was valid, but it returns that for everything.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUTI1ABKGRvpm+kYGRoYQNhAhcUyQOEamcI6pUqwO1AiYSmNkbZaEzTAyQJhhBJU0QFZpRoQZSO4whqpE0WZOhF8g7gA5wdQUmxkWhM0AOiMWAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Index = _t, CW = _t, Start_CW = _t, END_CW = _t, Valid_from = _t, Valid_to = _t, Value = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Valid_from", type date}, {"Valid_to", type date}, {"Value", Int64.Type}, {"Start_CW", type date}, {"END_CW", type date}}),
    GroupedSum =
        Table.AddColumn(
            #"Changed Type",
            "Grouped Sum",
            each
            List.Sum(
                Table.SelectRows(
                    #"Changed Type",
                    each [Start_CW] >= [Valid_from] and [END_CW] <= [Valid_to]
                )[Value]
            )
        )
in
    GroupedSum

But I am sure that is not what you want.

Can you provide some valid data, as well as an example of the expected output?

By the way, this will NOT perform well on large datasets. Power Query isn't set up to scan tables like this. A few hundred rows will be no problem A few thousand will be noticable. Over 10K-15K records and it will be pain. Over 100K and it won't finish. This is best done in DAX.

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