What Function should I use for Row Sum?

Hi bensim1973 ,

Please refer:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WclTSUTI0ghCxOtFKTkCmiSmQMLME851BTHMIAeK7AJlGJkDCAMx1BbJMjSAEiO8G0w4kYmMB", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Part = _t, #"Sales Qty" = _t, #"Expected Sales Qty" = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Part", type text}, {"Sales Qty", Int64.Type}, {"Expected Sales Qty", Int64.Type}}),
    #"Added Custom" = Table.AddColumn(#"Changed Type", "Custom", each if [Part]="B" then List.Sum(Table.SelectRows(#"Changed Type", each List.Contains({"B","D"},[Part]))[Sales Qty])
else if [Part]="D" then 0 else [Sales Qty])
in
    #"Added Custom"

Output:

Best Regards,
Gao
Community Support Team

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Reference to a new table, filter it down to D. Add a dummy column "B". Merge it back in matching part column to dummy column. Then everything is on the right rows for you to do a simple custom column if part = D then 0 else expected sales qty + D.sales.