public function toString()
 {
     if ($this->from->isEmpty()) {
         return '';
     }
     return "UPDATE " . $this->from->toString() . $this->set->toString() . $this->where->toString() . $this->limit->toString();
 }
 public function equals(Gpf_SqlBuilder_FromClause $from)
 {
     return $from->toString() == $this->toString();
 }
 public function toString()
 {
     return "DELETE " . $this->delete->toString() . "FROM " . $this->from->toString() . $this->where->toString() . $this->limit->toString();
 }
 public function toString()
 {
     return $this->select->toString() . ($this->from->isEmpty() ? '' : "FROM " . $this->from->toString()) . $this->where->toString() . $this->groupBy->toString() . $this->having->toString() . $this->orderBy->toString() . $this->limit->toString();
 }