我想上传一些图片文件到我的服务器上，并在mysql数据库中记录文件名，但不知何故存在一个问题

<form action="/upload" method="post" enctype="multipart/form-data">
  <input type="file" name="file" accept="image/*">
  <input type="submit" value="Upload">
</form>

const express = require('express');
const multer = require('multer');
const app = express();
const upload = multer({ dest: 'uploads/' });

app.post('/upload', upload.single('file'), (req, res) => {
  if (!req.file) {
    return res.status(400).send('No file uploaded.');
  }
  const fileName = req.file.filename;
  // 记录文件名到数据库
  saveFileNameToDatabase(fileName);
  res.send('File uploaded successfully.');
});

function saveFileNameToDatabase(fileName) {
  // 连接数据库并插入文件名
  const mysql = require('mysql');
  const connection = mysql.createConnection({
    host: 'localhost',
    user: 'user',
    password: 'password',
    database: 'database_name'
  });
  connection.connect();
  const sql = `INSERT INTO files (filename) VALUES (?)`;
  connection.query(sql, [fileName], (error, results) => {
    if (error) throw error;
    console.log('File name saved to database.');
  });
  connection.end();
}

app.listen(3000, () => {
  console.log('Server started on port 3000');
});

CREATE TABLE files (
  id INT AUTO_INCREMENT PRIMARY KEY,
  filename VARCHAR(255) UNIQUE NOT NULL
);

const uniqueFilename = `${Date.now()}-${Math.random().toString(36).substr(2, 9)}${req.file.originalname}`;

const mysql = require('mysql');
const connection = mysql.createConnection({
  host: 'localhost',
  user: 'user',
  password: 'password',
  database: 'database_name'
});
connection.connect((err) => {
  if (err) {
    console.error('Error connecting to database: ', err);
    return;
  }
  console.log('Connected to database.');
});