How to extract all the column names from a 'Table' data Type Column ?

Hi, Sayri , it's easy to extract all columns' names from each table in your scenario; you might add applied steps like this,

#"Column Names" = Table.TransformColumns(#"Previous Step", {{"Data", Table.ColumnNames}}),
#"Expanded Data" = Table.ExpandListColumn(#"Column Names", "Data")

Hello Sayri 

you can transform you tables first to get the first row of your tables as new table with Table.First and Record.ToTable.

Then expand the newly created table. here an easy example

let
    Source = #table({"Index", "Data"}, {{"Index0", #table({"ColumnA", "ColumnB"}, {{"A", "B"}, {"C", "D"}})}, {"Index1", #table({"Column1", "Column2"}, {{"E", "F"}, {"G", "H"}})}}),
    GetFirstRowOfTable = Table.TransformColumns
    (
        Source,
        {
            {
                "Data",
                each Record.ToTable(Table.First(_))
            }
        }
    ),
    #"Expanded Data" = Table.ExpandTableColumn(GetFirstRowOfTable, "Data", {"Name", "Value"}, {"Column name", "First row value"})
in
    #"Expanded Data"

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