docs: def select_batch(self, batch, batch_col_name=None):
          """selects the rows  in column batch_col_number
          (default: DbSheetCols.batch)"""
  
          if not batch_col_name:
              batch_col_name = self.db_sheet_cols.batch
          logger.debug("selecting batch - %s" % batch)
          sheet = self.table
          identity = self.db_sheet_cols.id
          exists_col_number = self.db_sheet_cols.exists
  
          criterion = sheet.loc[:, batch_col_name] == batch
          exists = sheet.loc[:, exists_col_number] > 0
          # This will crash if the col is not of dtype number
          sheet = sheet[criterion & exists]
          return sheet.loc[:, identity].values.astype(int)