public function replaceColumn($oldColumnAlias, $columnName, $columnAlias = '', $tablePrefix = '')
 {
     if (array_key_exists($oldColumnAlias, $this->clause)) {
         $column = new Gpf_SqlBuilder_SelectColumn($columnName, $columnAlias);
         $column->setTablePrefix($tablePrefix);
         $this->clause[$oldColumnAlias] = $column;
     }
 }
 private function getHeaderColumn(Gpf_SqlBuilder_SelectColumn $selectColumn)
 {
     return array($this->encodeColumn($selectColumn->getAlias()), $selectColumn->getName());
 }