this.currentOrder?.id ?? null;

这句话是一个JavaScript表达式，它使用了短路求值（Short-Circuit Evaluation）和条件运算符（?.）。`this.currentOrder` 可能是个对象，表示当前订单，`.` 后面的 `.id` 表示访问这个对象的 `id` 属性。

`?.` 运算符是 null 安全操作符，如果 `this.currentOrder` 为null或者undefined，那么整个表达式的结果将是 `null`；如果 `this.currentOrder` 存在并且其 `id` 属性也存在，那么返回 `this.currentOrder.id` 的值。如果省略 `?? null`，则当 `currentOrder` 为空时会抛出错误，而这里加了 `?? null` 表示默认值为 `null`，避免了这种错误。

相关问题

Node.js+MySQL实现的客服系统

以下是一个简单的Node.js和MySQL实现的客服系统的示例代码：

1. 创建数据库和表

首先，需要创建一个MySQL数据库和相应的表，用于存储客户数据和订单信息。可以执行以下SQL语句来创建表：

CREATE DATABASE `customer_service`;

USE `customer_service`;

CREATE TABLE `customers` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) NOT NULL,
  `email` varchar(255) NOT NULL,
  `phone` varchar(20) NOT NULL,
  `message` text NOT NULL,
  `created_at` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

CREATE TABLE `orders` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `customer_id` int(11) NOT NULL,
  `product` varchar(255) NOT NULL,
  `quantity` int(11) NOT NULL,
  `price` decimal(10,2) NOT NULL,
  `created_at` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY (`id`),
  KEY `fk_customer_id` (`customer_id`),
  CONSTRAINT `fk_customer_id` FOREIGN KEY (`customer_id`) REFERENCES `customers` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

2. 创建Node.js服务端程序

接下来，可以创建一个Node.js服务端程序，用于处理客户端的请求和响应。可以使用Express框架来创建一个简单的HTTP服务，并使用mysql模块来连接MySQL数据库和执行SQL查询。以下是一个示例程序：

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

const app = express();

const connection = mysql.createConnection({
  host: 'localhost',
  user: 'root',
  password: 'password',
  database: 'customer_service'
});

connection.connect((err) => {
  if (err) {
    console.error('Error connecting to database: ' + err.stack);
    return;
  }

  console.log('Connected to database with ID: ' + connection.threadId);
});

app.use(express.json());

app.get('/customers', (req, res) => {
  connection.query('SELECT * FROM customers', (err, results) => {
    if (err) {
      console.error('Error executing query: ' + err.stack);
      res.status(500).send('Error executing query');
      return;
    }

    res.send(results);
  });
});

app.post('/customers', (req, res) => {
  const { name, email, phone, message } = req.body;

  connection.query('INSERT INTO customers (name, email, phone, message) VALUES (?, ?, ?, ?)', [name, email, phone, message], (err, results) => {
    if (err) {
      console.error('Error executing query: ' + err.stack);
      res.status(500).send('Error executing query');
      return;
    }

    res.send(results);
  });
});

app.get('/orders', (req, res) => {
  connection.query('SELECT * FROM orders', (err, results) => {
    if (err) {
      console.error('Error executing query: ' + err.stack);
      res.status(500).send('Error executing query');
      return;
    }

    res.send(results);
  });
});

app.post('/orders', (req, res) => {
  const { customer_id, product, quantity, price } = req.body;

  connection.query('INSERT INTO orders (customer_id, product, quantity, price) VALUES (?, ?, ?, ?)', [customer_id, product, quantity, price], (err, results) => {
    if (err) {
      console.error('Error executing query: ' + err.stack);
      res.status(500).send('Error executing query');
      return;
    }

    res.send(results);
  });
});

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

3. 创建Vue客户端程序

最后，可以创建一个Vue客户端程序，用于展示用户界面和发送HTTP请求。可以使用axios模块来发送HTTP请求，并使用Vue组件来创建用户界面。以下是一个示例程序：

<template>
  <div>
    <h2>Customer List</h2>

    <table>
      <thead>
        <tr>
          <th>Name</th>
          <th>Email</th>
          <th>Phone</th>
          <th>Message</th>
          <th>Date</th>
        </tr>
      </thead>
      <tbody>
        <tr v-for="customer in customers" :key="customer.id">
          <td>{{ customer.name }}</td>
          <td>{{ customer.email }}</td>
          <td>{{ customer.phone }}</td>
          <td>{{ customer.message }}</td>
          <td>{{ customer.created_at }}</td>
        </tr>
      </tbody>
    </table>

    <h2>Order List</h2>

    <table>
      <thead>
        <tr>
          <th>Customer</th>
          <th>Product</th>
          <th>Quantity</th>
          <th>Price</th>
          <th>Date</th>
        </tr>
      </thead>
      <tbody>
        <tr v-for="order in orders" :key="order.id">
          <td>{{ customers.find(c => c.id === order.customer_id).name }}</td>
          <td>{{ order.product }}</td>
          <td>{{ order.quantity }}</td>
          <td>{{ order.price }}</td>
          <td>{{ order.created_at }}</td>
        </tr>
      </tbody>
    </table>

    <form @submit.prevent="submit">
      <h2>Contact Us</h2>

      <div>
        <label>Name:</label>
        <input type="text" v-model="name" required>
      </div>

      <div>
        <label>Email:</label>
        <input type="email" v-model="email" required>
      </div>

      <div>
        <label>Phone:</label>
        <input type="tel" v-model="phone" required>
      </div>

      <div>
        <label>Message:</label>
        <textarea v-model="message" required></textarea>
      </div>

      <button type="submit">Submit</button>
    </form>
  </div>
</template>

<script>
import axios from 'axios';

export default {
  data() {
    return {
      customers: [],
      orders: [],
      name: '',
      email: '',
      phone: '',
      message: ''
    };
  },

  mounted() {
    axios.get('/customers').then(response => {
      this.customers = response.data;
    });

    axios.get('/orders').then(response => {
      this.orders = response.data;
    });
  },

  methods: {
    submit() {
      axios.post('/customers', { name: this.name, email: this.email, phone: this.phone, message: this.message }).then(response => {
        alert('Your message has been sent.');
        this.name = '';
        this.email = '';
        this.phone = '';
        this.message = '';
      });
    }
  }
};
</script>

以上仅是一个简单的示例代码，实际的客服系统可能要更加复杂和完善，需要根据实际业务需求进行设计和开发。

请根据以下sql生成Java实体类：CREATE TABLE `pl_write_off_recharge_record` ( `id` varchar(40) NOT NULL COMMENT '主键id', `recharge_order_id` varchar(40) NOT NULL DEFAULT '' COMMENT '充值订单号', `write_off_order_id` varchar(40) NOT NULL COMMENT '核销订单号-消费退款提现的订单号', `trans_time` datetime NOT NULL COMMENT '交易时间', `trans_type` int(11) NOT NULL DEFAULT '0' COMMENT '交易类型：0-消费，2-提现，3-退款', `wallet_type` int(11) NOT NULL COMMENT '充值钱包类型：0-门店钱包，2-平台钱包', `store_id` varchar(40) NOT NULL DEFAULT '' COMMENT '门店id', `store_name` varchar(40) NOT NULL DEFAULT '' COMMENT '门店名称', `order_amount` decimal(10,2) NOT NULL DEFAULT '0.00' COMMENT '订单金额', `write_off_amount` decimal(10,2) NOT NULL DEFAULT '0.00' COMMENT '核销金额', `write_off_recharge_amount` decimal(10,2) NOT NULL DEFAULT '0.00' COMMENT '核销充值金额', `write_off_give_amount` decimal(10,2) NOT NULL DEFAULT '0.00' COMMENT '核销赠送金额', `wait_write_off_amount` decimal(10,2) NOT NULL DEFAULT '0.00' COMMENT '待核销金额', `create_by` varchar(40) DEFAULT '' COMMENT '创建人', `create_time` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '创建时间', `update_by` varchar(40) DEFAULT NULL COMMENT '更新人', `update_time` datetime DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT '最后一次更新时间', `remark` varchar(100) DEFAULT '' COMMENT '备注', PRIMARY KEY (`id`) USING BTREE ) ENGINE=InnoDB DEFAULT CHARSET=utf8 ROW_FORMAT=DYNAMIC COMMENT='充值核销记录';

package com.example.demo.entity;

import java.math.BigDecimal;
import java.util.Date;

public class PlWriteOffRechargeRecord {

private String id;
private String rechargeOrderId;
private String writeOffOrderId;
private Date transTime;
private int transType;
private int walletType;
private String storeId;
private String storeName;
private BigDecimal orderAmount;
private BigDecimal writeOffAmount;
private BigDecimal writeOffRechargeAmount;
private BigDecimal writeOffGiveAmount;
private BigDecimal waitWriteOffAmount;
private String createBy;
private Date createTime;
private String updateBy;
private Date updateTime;
private String remark;

public String getId() {
return id;
}

public void setId(String id) {
this.id = id;
}

public String getRechargeOrderId() {
return rechargeOrderId;
}

public void setRechargeOrderId(String rechargeOrderId) {
this.rechargeOrderId = rechargeOrderId;
}

public String getWriteOffOrderId() {
return writeOffOrderId;
}

public void setWriteOffOrderId(String writeOffOrderId) {
this.writeOffOrderId = writeOffOrderId;
}

public Date getTransTime() {
return transTime;
}

public void setTransTime(Date transTime) {
this.transTime = transTime;
}

public int getTransType() {
return transType;
}

public void setTransType(int transType) {
this.transType = transType;
}

public int getWalletType() {
return walletType;
}

public void setWalletType(int walletType) {
this.walletType = walletType;
}

public String getStoreId() {
return storeId;
}

public void setStoreId(String storeId) {
this.storeId = storeId;
}

public String getStoreName() {
return storeName;
}

public void setStoreName(String storeName) {
this.storeName = storeName;
}

public BigDecimal getOrderAmount() {
return orderAmount;
}

public void setOrderAmount(BigDecimal orderAmount) {
this.orderAmount = orderAmount;
}

public BigDecimal getWriteOffAmount() {
return writeOffAmount;
}

public void setWriteOffAmount(BigDecimal writeOffAmount) {
this.writeOffAmount = writeOffAmount;
}

public BigDecimal getWriteOffRechargeAmount() {
return writeOffRechargeAmount;
}

public void setWriteOffRechargeAmount(BigDecimal writeOffRechargeAmount) {
this.writeOffRechargeAmount = writeOffRechargeAmount;
}

public BigDecimal getWriteOffGiveAmount() {
return writeOffGiveAmount;
}

public void setWriteOffGiveAmount(BigDecimal writeOffGiveAmount) {
this.writeOffGiveAmount = writeOffGiveAmount;
}

public BigDecimal getWaitWriteOffAmount() {
return waitWriteOffAmount;
}

public void setWaitWriteOffAmount(BigDecimal waitWriteOffAmount) {
this.waitWriteOffAmount = waitWriteOffAmount;
}

public String getCreateBy() {
return createBy;
}

public void setCreateBy(String createBy) {
this.createBy = createBy;
}

public Date getCreateTime() {
return createTime;
}

public void setCreateTime(Date createTime) {
this.createTime = createTime;
}

public String getUpdateBy() {
return updateBy;
}

public void setUpdateBy(String updateBy) {
this.updateBy = updateBy;
}

public Date getUpdateTime() {
return updateTime;
}

public void setUpdateTime(Date updateTime) {
this.updateTime = updateTime;
}

public String getRemark() {
return remark;
}

public void setRemark(String remark) {
this.remark = remark;
}
}