protected function pushValues()
    {
        if( empty($this->values) ) {
            throw new LogicException('No values specified');
        }
        foreach( $this->values as $key => $value ) {
            if( !is_int($key) ) {
                $this->parts[] = $this->quoteIdentifierIfNotExpression($key);
                $this->parts[] = '=';
            }
            if( $value instanceof Expression ) {
                $this->parts[] = (string) $value;
            } else if( !is_int($key) ) {
                $this->parts[] = '?';
                $this->params[] = $value;
            }
            $this->parts[] = ',';
        }
        array_pop($this->parts);
    }