如何在module.export函数中使用回调进行mysql查询-- nodejs，express

npm install mysql express

const mysql = require('mysql');
const express = require('express');
const app = express();

const pool = mysql.createPool({
  host: 'localhost',
  user: 'your_username',
  password: 'your_password',
  database: 'your_database'
});

module.exports = {
  query: function(sql, callback) {
    pool.getConnection(function(err, connection) {
      if (err) {
        callback(err, null);
      } else {
        connection.query(sql, function(err, results) {
          connection.release();
          callback(err, results);
        });
      }
    });
  }
};

const db = require('./db'); // 假设查询函数定义在名为db.js的文件中

app.get('/users', function(req, res) {
  const sql = 'SELECT * FROM users';
  db.query(sql, function(err, results) {
    if (err) {
      res.status(500).send('Error retrieving users');
    } else {
      res.json(results);
    }
  });
});

app.listen(3000, function() {
  console.log('Server is running on port 3000');
});