Adding columns

Hi Dicken 

Your Power Query (M) script for reordering and standardizing table columns is already quite efficient and idiomatic, especially given the use of Table.ReorderColumns with MissingField.UseNull to handle missing columns gracefully. This approach is beneficial when you want to enforce a consistent column schema across multiple tables, regardless of the input column order or presence.

In terms of simplification, this script is already minimal and readable. You define your expected schema (allcol), and then reorder the columns while automatically inserting any missing ones with nulls. There's no significant way to make it “more efficient” in performance or syntax without sacrificing clarity. One minor change could be directly embedding the allcol list inside the Table.ReorderColumns call to eliminate an extra step, like this:

let
    Source = #table(type table [One = Text.Type, Five = Text.Type, Three = Text.Type],
                    {{"a", "b", "b"}, {"b", "c", "d"}, {"d", "a", "c"}}),
    Custom = Table.ReorderColumns(Source, {"One", "Two", "Three", "Four", "Five"}, MissingField.UseNull)
in
    Custom

This avoids a variable assignment but functionally behaves the same. Otherwise, your current approach is robust and suitable for use in standardized Power Query workflows, especially when normalizing column layouts for appending or merging tables.