问题

我正在开发一个 Discord.js 机器人，所以我在 Node.js 中工作。我需要做的是在我的 MySQL 表中搜索并获取所有满足某个条件的行（我可以做到），然后返回所有满足该条件的行的第一列的值。我尝试了多种不同的方法，但我的主要问题是我无法返回该值。我可以在查询中使用 `console.log` 打印它，但我真的需要返回它。

```js
function fetchOrders(status){
    textr = con.query(`SELECT * FROM orders WHERE OStatus = '${status}';`, function(err,rows) {
                if(err){
                    throw err
                }
                var text = "";
                if(rows.length<1){
                    text = "None"
                }else{
                    for(i=0;i<rows.length;i++){
                        text = text + "`" + rows[i].OID.toString() + "`, ";
                    }
                }
                if(text===""){
                    text = "None"
                }
                return text;
            })
    return textr    
}

text 的值是我需要的（直到 return text;，它工作得很完美），但当我返回它时，textr 不是 text。如果有人告诉我如何从查询中获取 text，我将非常感激。

<details>
<summary>英文:</summary>

I am working on a discord.js bot and so I am working in Node.Js. What I need to do is search through my MySQL table and get all rows that meet a criteria (which I can do) and then return the value in first column of all those rows meeting said criteria. I have tried multiple different methods, but my main issue is I can&#39;t return that value. I can `console.log` it from within my query, but I really need to return it.

function fetchOrders(status){
textr = con.query(SELECT * FROM orders WHERE OStatus = '${status}';, function(err,rows) {
if(err){
throw err
}
var text = "";
if(rows.length<1){
text = "None"
}else{
for(i=0;i<rows.length;i++){
text = text + "" + rows[i].OID.toString() + ", "
}
}
if(text===""){
text = "None"
}
return text;
})
return textr
}

The value `text` is what I need (up to `return text;` it works perfectly), but when I return it `textr` is not `text`. If someone could tell me how to get `text` out of my query, I would be very grateful.

</details>


# 答案1
**得分**: 0

我假设你对Node.js中的异步调用不太熟悉。接受回调函数的调用通常是异步执行的，这意味着调用的结果不会立即可用（即使在函数调用结束后也不会），但一旦结果可用，回调函数就会被调用。

你可以修改你的代码，接受另一个回调函数，这样你的函数有机会处理查询结果，然后通知调用者：

```javascript
function fetchOrders(status, callback) {
    con.query(`SELECT * FROM orders WHERE OStatus = '${status}';`, function(err, rows) {
        if (err) {
            callback(err);
        }
        var text = "";
        if (rows.length < 1) {
            text = "None";
        } else {
            for (i = 0; i < rows.length; i++) {
                text = text + "`" + rows[i].OID.toString() + "`, ";
            }
        }
        if (text === "") {
            text = "None";
        }

        callback(null, text);
    })
}

请注意，这些函数不再返回值，而是将值（或错误）传递给回调函数。这也需要更改调用此函数的代码，所以不再是：

var result = fetchOrders(status);

而是需要写成：

fetchOrders(status, function (err, result) {
    if (err) {
        throw err;
    }
    
    // 对结果进行处理
})

function fetchOrders(status, callback){
    con.query(`SELECT * FROM orders WHERE OStatus = '${status}';`, function(err,rows) {
        if(err){
            callback(err);
        }
        var text = "";
        if(rows.length<1){
            text = "None"
        }else{
            for(i=0;i<rows.length;i++){
                text = text + "`" + rows[i].OID.toString() + "`, "
            }
        }
        if(text===""){
            text = "None"
        }

        callback(null, text);
    })
}

var result = fetchOrders(status);

fetchOrders(status, function (err, result) {
    if (err) {
        throw err;
    }
    
    // do something with result
})

答案2

得分: 0

将这个函数转换成 promise 风格，就像下面这样：

function fetchOrders(status) {
  return new Promise((resolve, reject) => {
    con.query(`SELECT * FROM orders WHERE OStatus = '${status}';`, function (
      err,
      rows
    ) {
      if (err) {
        reject(err);
      }
      var text = "";
      if (rows.length < 1) {
        text = "None";
      } else {
        for (i = 0; i < rows.length; i++) {
          text = text + '`' + rows[i].OID.toString() + '`, ';
        }
      }
      if (text === "") {
        text = "None";
      }
      resolve(text);
    });
  });
}

然后像这样调用它：

fetchOrders(status)
  .then(result => {
    console.log(result);
  })

或者使用 async/await：

const response = await fetchOrders(status);

function fetchOrders(status) {
  return Promise((resolve, reject) => {
    con.query(`SELECT * FROM orders WHERE OStatus = '${status}';`, function(
      err,
      rows
    ) {
      if (err) {
        reject(err);
      }
      var text = "";
      if (rows.length < 1) {
        text = "None";
      } else {
        for (i = 0; i < rows.length; i++) {
          text = text + "`" + rows[i].OID.toString() + "`, ";
        }
      }
      if (text === "") {
        text = "None";
      }
      resolve(text);
    });
  });
}

fetchOrders
  .then(result => {
    console.log(result);
  })

const response = await fetchOrders();