Replace duplicated values with null in the same column per a given list of columns

[davidleal]

I found several solutions for a similar problem, but it creates an additional column or does find and replace value which is not what I need. Let's say I have a Name column with the following values:

name1
name1
name2
name2
name1

and I would like to have the following output (the same previous column but with some values transformed):

name1
null
name2
null
null

In this question: Testing a query function to replace duplicated values with null, but I am getting errors  @wdx223_Daniel provided the following solution that returns a list with the expected output:

let
  Source = Excel.CurrentWorkbook(){[Name="TB_source"]}[Content],
  #"Changed Type" = Table.TransformColumnTypes(Source,{{"Name", type text}, {"Amount", Int64.Type}}),
  RemoveDups = (lst as list)=>List.Accumulate(lst,{},(x,y)=> x&{ if List.Contains(x,y) then null else y}),
  replaceValues = RemoveDups(Table.Column(#"Changed Type", "Name"))
in
  replaceValues

but I was not able to assign this list to the existing column Name. The question: Find and Replace text string in one table using value from another table does a find and replace, I was trying to customize it, but it doesn't work because for every value in the Find will be replaced wil null. Because it does a search (not by index position).

What I would need is to loop for all values of the a given column and replace with the corresponding value at the same index position of #"uniqueList". In other languages it would be a simple assignment: a=b, but I was not able to find a way in Power Query.

I was trying the following for example:

Table.ReplaceValue(#"Changed Type", each [Name], each replaceValues,Replacer.ReplaceValue,{"Name"})

but it return a the list of value on each row.

For the general case I would like to do the following transformation in #"Changed Type": For a list of selected columns replace  per column duplicated values with null. 

Thanks

[ImkeF]

Hi @davidleal ,

not with the native Table.ReplaceValue or Table.TransformColumns- functions, as they don't support recursion.

[davidleal]

Thanks, it works, so it looks like that the only we to do it is to recreate a new table again, but not replacing the existing column values with the new value list. 

[ImkeF · Accepted solution]

Hi @davidleal ,
utilizing on what you have already, I would suggest the following approach:

let
    Source = Table.FromRows(
        Json.Document(
            Binary.Decompress(
                Binary.FromText(
                    "i45W8kvMTTVU0lFyziypBNGGBkqxOpjCRghhI6gwiDbGLmyCKWwMpE2BwrEA",
                    BinaryEncoding.Base64
                ),
                Compression.Deflate
            )
        ),
        let
            _t = ((type nullable text) meta [Serialized.Text = true])
        in
            type table [Name = _t, City = _t, Amount = _t]
    ),
    #"Changed Type" = Table.TransformColumnTypes(
        Source,
        {{"Name", type text}, {"City", type text}, {"Amount", Int64.Type}}
    ),
    ListOfColumns = {"Name", "City"},
    RemainingColumns = List.Difference(Table.ColumnNames(#"Changed Type"), ListOfColumns),
    RemoveDups = (lst as list) =>
        List.Accumulate(lst, {}, (x, y) => x & {if List.Contains(x, y) then null else y}),
    replaceValues = List.Transform(ListOfColumns, each RemoveDups(Table.Column(#"Changed Type", _))),
    Custom1 = Table.FromColumns(
        replaceValues & Table.ToColumns(Table.SelectColumns(#"Changed Type", RemainingColumns)),
        ListOfColumns & RemainingColumns
    )
in
    Custom1