/**
  * sql version
  *
  * @return string
  */
 public function to_sql()
 {
     $column_sql = sprintf("%s %s", $this->_adapter->identifier($this->name), $this->sql_type());
     $column_sql .= $this->_adapter->add_column_options($this->type, $this->_options);
     return $column_sql;
 }