Return original columns after grouping

Hi Anonymous 

Please see the below M expression, if you have any questions let me know.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("bYzBDcAgDAN38RuJOJS2mSVi/zWg9Bfys0+ncwdRIKxiVYW2DjGKQyNuG7dP6Id95ZG+lp72nbefvP3GiG5sMfJjSsLHBA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type text) meta [Serialized.Text = true]) in type table [Index = _t, Date = _t, #"Member ID" = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Index", Int64.Type}, {"Date", type date}, {"Member ID", Int64.Type}}),
    #"Reordered Columns" = Table.ReorderColumns(#"Changed Type",{"Member ID", "Date", "Index"}),
    #"Grouped Rows" = Table.Group(#"Reordered Columns", {"Member ID"}, {{"Date", each List.Max([Date]), type date}, {"Index", each Table.FirstN( Table.Sort( _, {"Date", Order.Descending} ), 1 )[Index], type list}}),
    #"Extracted Values" = Table.TransformColumns(#"Grouped Rows", {"Index", each Text.Combine(List.Transform(_, Text.From)), Int64.Type})
in
    #"Extracted Values"

Best Regards,
Mariusz

If this post helps, then please consider Accepting it as the solution.

Please feel free to connect with me.

Anonymous  - The solution that HotChilli  is a good one, if the indexes are in the required order. Otherwise, you can Merge Tables to the step prior to the Group By - something like this (Note: "Expanded Partition" is the step prior to "Grouped Rows" step:

    #"Grouped Rows" = Table.Group(#"Expanded Partition", {"ProductId"}, {{"max_date", each List.Max([SalesDateId]), type number}}),
    #"Merged Queries" = Table.NestedJoin(#"Grouped Rows", {"ProductId", "max_date"}, #"Expanded Partition", {"ProductId", "SalesDateId"}, "Product", JoinKind.Inner),
    #"Expanded Product" = Table.ExpandTableColumn(#"Merged Queries", "Product", {"Index"}, {"Index"})
in
    #"Expanded Product"

I hope this helps. If it does, please Mark as a solution.
I also appreciate Kudos.