Power Query : customize column that count number of rows for each ROW

If I understand properly, you can do this in PQ by

• Group by Product Key
• Count the number of YES in each group
• Re-expand the table

Note that your example seems to be incorrect

• 31/2/2000 is not a valid date
• Key A3 only has a count of one (1), not three(3)

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WcjRU0lEystQ30jcyMDIAsiNdg5VidaAShkb6BoYgGUNkGSOQjCGSjJ8/RMIYZBZQwhhDiwlIxginjDG6TCwA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Product Key" = _t, Date = _t, Sold = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{
        {"Product Key", type text}, {"Date", type date}, {"Sold", type text}}, "en-150"),
    #"Grouped Rows" = Table.Group(#"Changed Type", {"Product Key"}, {
        {"All", each _, type table [Product Key=nullable text, Date=nullable date, Sold=nullable text]},
        {"Count", each List.Count(List.RemoveItems(_[Sold],{"NO"})), Int64.Type}
        }),
    #"Expanded All" = Table.ExpandTableColumn(#"Grouped Rows", "All", {"Date", "Sold"}, {"Date", "Sold"})
in
    #"Expanded All"

Data

Results

Sorry about that.

The new line should have been:

    {"Last Date Sold", (t)=> List.Max(Table.SelectRows(t, each [Sold] = "YES")[Date]), type date}

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WcjRU0lEystQ30jcyMDIAsiNdg5VidaAShkb6BoYgGUNkGSOQjCGSjJ8/RMIYZBZQwhhDiwlIxginjDG6TCwA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Product Key" = _t, Date = _t, Sold = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{
        {"Product Key", type text}, {"Date", type date}, {"Sold", type text}}, "en-150"),
    #"Grouped Rows" = Table.Group(#"Changed Type", {"Product Key"}, {
        {"All", each _, type table [Product Key=nullable text, Date=nullable date, Sold=nullable text]},
        {"Count", each List.Count(List.RemoveItems(_[Sold],{"NO"})), Int64.Type},
        {"Last Date Sold", (t)=> List.Max(Table.SelectRows(t, each [Sold] = "YES")[Date]), type date}
        }),
    #"Expanded All" = Table.ExpandTableColumn(#"Grouped Rows", "All", {"Date", "Sold"}, {"Date", "Sold"})
in
    #"Expanded All"