GdaQueryField Parent class for any query's field @query: @node: @error: @Returns: @orig: @Returns: @query: @sqlfield: @error: @Returns: @qfield: @alias: @qfield: @Returns: @qfield: @visible: @qfield: @Returns: @qfield: @internal: @qfield: @Returns: @qfield: @Returns: @qfield: @Returns: @qfield1: @qfield2: @Returns: @qfield: @Returns: