Difference between two line entries with the same ID

let
    s = your_table,
    rows = List.Buffer(Table.ToRecords(s)),
    add_col = List.Generate(
        () => [i = 0, val = rows{0} & [ValueColumn = 0]],
        (x) => rows{x[i]}? <> null,
        (x) => [i = x[i] + 1, val = rows{i} & [ValueColumn = rows{i}[ValueMax] - x[val][ValueMax]]],
        (x) => x[val]
    ),
    z = Table.FromRecords(add_col)
in
    z

Hi JohannesBier 

You can refer to the following solution.

Sample data

First step

Add index column group by the id and then expand it., you can refer to the following code or the video.

Insert a new step

= Table.Group(#"Changed Type"(the last step name), {"ID"}, {{"Data", each Table.AddIndexColumn(_,"Index",1,1),type table}})

Create Row Number for Each Group in Power BI using Power Query - RADACAD

Second Step:Add a custom column to calculate the difference

=[ValueMax]-List.Min(Table.SelectRows(#"Expanded Data"(last step name),(x)=>x[ID]=[ID] and x[Index]=[Index]-1)[ValueMax])

Output

And you can put the following code to advanved editor in power query to refer to 

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUTI0UIrVgTCNEExjBNMEwjQCMk0RTCRRMwTTHMG0ADJjAQ==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [ID = _t, ValueMax = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"ID", Int64.Type}, {"ValueMax", Int64.Type}}),
    #"Grouped Rows" = Table.Group(#"Changed Type", {"ID"}, {{"Data", each Table.AddIndexColumn(_,"Index",1,1),type table}}),
    #"Expanded Data" = Table.ExpandTableColumn(#"Grouped Rows", "Data", {"ValueMax", "Index"}, {"ValueMax", "Index"}),
    #"Added Custom" = Table.AddColumn(#"Expanded Data", "Difference", each [ValueMax]-List.Min(Table.SelectRows(#"Expanded Data",(x)=>x[ID]=[ID] and x[Index]=[Index]-1)[ValueMax]))
in
    #"Added Custom"

Best Regards!

Yolo Zhu

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