code: public function compile()
  	{
  		if (empty($this->clauses))
  		{
  			return "";
  		}
  
  		$sql = array();
  		foreach ($this->clauses as $clause)
  		{
  			list($column, $direction) = $clause;
  
  			if ($direction === null)
  			{
  				$sql[] = $column;
  			}
  			else
  			{
  				$sql[] = "{$column} {$direction}";
  			}
  		}
  
  		return $this->getType()." ".join(", ", $sql);
  	}
docs: Compiles the query into raw SQL
  
  @return  string