Create a string Column in Advanced Editor

Sorry... But you are all incorrect.  I had a different typo.  It does work to assign a "string" to a new column in a select statemement:

let
    Source = Sql.Database("xxxx", "yyy", [Query="
        select
              'GDC' as ""Type""
        from
            [table1] 

        union
        select
            'bill' as [Type]
        from
            [table2] bill
      "])
in
    Source