public function onDuplicateKeyUpdate($key, $value = null)
    {
        if( func_num_args() == 2 ) {
            $this->onDuplicateKeyUpdate[$key] = $value;
        } else if( $key instanceof Expression ) {
            $this->onDuplicateKeyUpdate[] = $key;
        } else if( is_array($key) ) {
            $this->onDuplicateKeyUpdate = $key;
        }
        return $this;
    }