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吴恩达2022机器学习专项课程(一) 3.4 成本函数的直观理解

code_balance 03-25 08:00 阅读 3
nodeJs中实现连表查询
router.post('/getOrder', async function(req, res, next) {
    let userId = req.body.phone;

    let sql = 'select * from orders where userId=?';
    let orders = await new Promise((resolve, reject) => {
        connection.query(sql, [userId], function(error, result) {
            if (error) {
                reject(error);
            } else {
                resolve(result);
            }
        });
    });

    for (let i = 0; i < orders.length; i++) {
        let foodId = orders[i].foodId;
        let foodSql = 'select * from food where id=?';
        let food = await new Promise((resolve, reject) => {
            connection.query(foodSql, [foodId], function(error, result) {
                if (error) {
                    reject(error);
                } else {
                    resolve(result);
                }
            });
        });
        orders[i].foodList = food; // 将foodList数组添加到对应的order对象中
    }

    console.log('Orders with Food List:', orders);

    res.send({
        code: 200,
        data: {
            data: orders
        }
    });
});

在这里插入图片描述

异步操作

创建一个Promise对象:

let myPromise = new Promise((resolve, reject) => {
    // 异步操作,比如数据库查询、文件读取等
    if (/* 操作成功 */) {
        resolve('操作成功,返回结果');
    } else {
        reject('操作失败,返回错误信息');
    }
});

myPromise.then((result) => {
    // 当Promise状态为fulfilled时调用,result为操作成功的结果
    console.log(result);
}).catch((error) => {
    // 当Promise状态为rejected时调用,error为操作失败的错误信息
    console.log(error);
});

let promise1 = Promise.resolve(1);
let promise2 = Promise.resolve(2);

Promise.all([promise1, promise2])
    .then((results) => {
        console.log(results); // [1, 2]
    })
    .catch((error) => {
        console.log(error);
    });

let promise1 = new Promise((resolve) => setTimeout(resolve, 500, 'one'));
let promise2 = new Promise((resolve) => setTimeout(resolve, 100, 'two'));

Promise.race([promise1, promise2])
    .then((value) => {
        console.log(value); // 'two'
    });

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