code: public function selectCol(string $sql, array $parameters = [])
      {
          $statement = $this->select($sql, $parameters);
  
          if ($statement === null) {
              return false;
          }
  
          $datas = $this->readAll($statement);
          $col = [];
          foreach ($datas as $data) {
              $col[] = \current($data);
          }
  
          $statement->closeCursor();
          $statement = null;
  
          return $col;
      }
docs: @param string $sql
  @param array  $parameters
  
  @throws DatabaseException
  
  @return array|bool