fill blank values with values in another column

Hello murali5431 

you can add a new column and make there a calculation like this

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjDUdSxN1zUyUNJBYsfqRCspgERMkUUMTJDUmqCrNUTojgUA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Actual arrival date" = _t, #"Expected date of Arrival" = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Actual arrival date", type date}, {"Expected date of Arrival", type date}}),
    #"Added Custom" = Table.AddColumn(#"Changed Type", "Actual arrival date new", each if [Actual arrival date]=null then [Expected date of Arrival] else [Actual arrival date]),
    #"Removed Columns" = Table.RemoveColumns(#"Added Custom",{"Actual arrival date"}),
    #"Renamed Columns" = Table.RenameColumns(#"Removed Columns",{{"Actual arrival date new", "Actual arrival date"}})
in
    #"Renamed Columns"

or you can use Table.TransformRows like this

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjDUdSxN1zUyUNJBYsfqRCspgERMkUUMTJDUmqCrNUTojgUA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Actual arrival date" = _t, #"Expected date of Arrival" = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Actual arrival date", type date}, {"Expected date of Arrival", type date}}),
    TransRow = Table.FromRecords(Table.TransformRows(#"Changed Type",(row)=> if row[Actual arrival date]= null then Record.TransformFields(row, {"Actual arrival date", each row[Expected date of Arrival]}) else row))
in
    TransRow

Copy paste this code to the advanced editor in a new blank query to see how the solution works.

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Have fun

Jimmy