如何从 Nodejs 中的循环中添加 MySQL 查询结果？

Question

Essentially, I have an object with string keys and values (ex. {"michigan":"minnesota"}). I'm trying to loop through all of these key value pairs and make a query from my database, and add the result to a list, which will then be what is returned to the front end.

var return_list = []

        Object.keys(obj).forEach(function(key){
            const state1 = key;
            const state2 = obj[key];

            const sql_select = 'SELECT column1,column2 from database WHERE state = ? OR state=?';
        
            db.query(sql_select,[state1,state2], (err,result) => {
                
                return_list.push(result);
            });
        })

This is what I have in simplest terms, and would like to send return_list back to the front end. The problem I'm running into is I can console.log the result within db.query call, but I can't push the result to the list or call it anywhere outside of the query. I'm fairly new to both front end and back end development, so any possible ideas would definitely be helpful!

Answer 1

The issue is because the database transaction is not instant, so you need to use either promises or async-await.

Using async await would be something like this (untested),

async function get_return_list () {
    var return_list = []

    Object.keys(obj).forEach(function(key){
        const state1 = key;
        const state2 = obj[key];
    
        const sql_select = 'SELECT column1,column2 from database WHERE state = ? OR state=?';
    
         await db.query(sql_select,[state1,state2], (err,result) => {
            
            return_list.push(result);
        });
    })

    return return_list
} 

see for more detail: https://eloquentjavascript.net/11_async.html

Answer 2

First, make sure you are working with mysql2 from npm. Which provides async method of mysql. Second, Note that when you query SELECT , you don't get the "real" result in first. Suppose you get result , then, the "real" results are held in result[0] .

(async () => {
    const promiseArr = [];

    for (const key of Object.keys(yourOBJ)) {
        const state1 = key;
        const state2 = yourOBJ[key];
        const sql_select = 'SELECT column1,column2 from database WHERE state = ? OR state=?';

        promiseArr.push(db.query(sql_select, [state1, state2]));
    }

    let results;

    try {
        results = await Promise.all(promiseArr);
    } catch (e) {
        throw '...';
    }

    const return_list = results.reduce((finalArray, item) => {
        finalArray = [
            ...finalArray,
            ...item[0],
        ]
    }, []);
})();

Answer 3

The problem is that the forEach returns void.
So you can't wait for the asynchronous code to run before you return it.
When we're dealing with an array of promises such as db queries ( like in your case ) or API calls, we should wait for every one of them to be executed.
That's when we use the Promise.all

Try doing it like this:

const queryResults = await Promise.all(
   Object.keys(obj).map(async (key) => {
    const state1 = key;
    const state2 = obj[key];

    const sql_select = 'SELECT column1,column2 from database WHERE state = ? OR state=?';

    return new Promise((resolve, reject) => 
      db.query(sql_select,[state1,state2], (err, result) => {
        if (err) 
          return reject(err)
        else
          return resolve(result)
      })
    )
  })
)
console.log('queryResults', queryResults)
// now you give this queryResults back to your FE

Small tips for your fresh start:

• never use var, try always use const or if needed, let.
• try always use arrow functions ( () => {...} ) instead of regular functions ( function () {...} ), It's hard to predict which scope this last one is using