Last 100 rows for each Unique value in Column

[waznybc]

The SQL table I'm pulling has millions of rows and I only need the last 100 records for each unique program in a column. See generic example below for pulling last 3 records for each unique program in a column:

Date	Program
1/1/2020	D
1/2/2020	A
1/3/2020	D
1/4/2020	A
1/5/2020	B
1/6/2020	B
1/7/2020	A
1/8/2020	D
1/9/2020	B
1/10/2020	D
1/11/2020	B
1/12/2020	D
1/13/2020	C
1/14/2020	B
1/15/2020	C
1/16/2020	C
1/17/2020	C
1/18/2020	C
1/19/2020	A
1/20/2020	A

I want to only return this:

Date	Program
1/20/2020	A
1/19/2020	A
1/7/2020	A
1/14/2020	B
1/11/2020	B
1/9/2020	B
1/18/2020	C
1/17/2020	C
1/16/2020	C
1/12/2020	D
1/10/2020	D
1/8/2020	D

[mahoneypat · Accepted solution]

Here is one way to do it by grouping your data, sorting and keep the last 3 dates for each.  To see how it works, just create a blank query, go to Advanced Editor, and replace the text there with the M code below that transforms your example data.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("Zc/JCcAwDETRXnQ2RFK8HrN0Ydx/GyFghzBzffwRdu9im22urhLklhFe8AXHhB2LiEVacE7ICAUnFY82nJhiYkaNU/O99loSaZWoySSFpJI0/JXrT8YD", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Date = _t, Program = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Date", type date}}),
    #"Grouped Rows" = Table.Group(#"Changed Type", {"Program"}, {{"AllRows", each _, type table [Date=nullable text, Program=nullable text]}}),
    #"Added Custom" = Table.AddColumn(#"Grouped Rows", "Last3Dates", each Table.FirstN(Table.Sort([AllRows], {{"Date", Order.Descending}}),3)),
    #"Removed Columns" = Table.RemoveColumns(#"Added Custom",{"AllRows"}),
    #"Expanded Last3Dates" = Table.ExpandTableColumn(#"Removed Columns", "Last3Dates", {"Date"}, {"Date"})
in
    #"Expanded Last3Dates"

Regards,

Pat