Fill empty cell with value contained within the same column but with a condition within another colum

Hello, RVIG

one of several options to solve this:

let
    Source = your_table,
    values = Table.SelectRows(Table.Distinct(Source[[parent_id], [DATA]]), each [DATA] <> null),
    dict = Record.FromList(values[DATA], List.Transform(values[parent_id], Text.From)),
    helper_column = 
        Table.AddColumn(
            Source, "helper",
            (x) => 
                if x[DATA] = null 
                then Record.FieldOrDefault(dict, Text.From(x[parent_id]), null)
                else x[DATA]
        )
in
    helper_column

then remove "DATA" column and rename "helper" as you want. Instead of exta (temporal) column one may choose to use Table.ReplaceValue (proved to be slower than other options) or a combination of Record.TransformFields and Table.TransformRows functions.