问为什么Node查询mysql结果为undefined？

var articleDetail;
const mysql =require('mysql');
//导入excel包
const excel =require('xlsx');
//导入express
const express =require('express');
const app =express();
app.get('/articleDetail',function (req, res){
    const id =req.query.id;
    console.log("id等于"+id)
    const db =mysql.createPool({
        host:"127.0.0.1",
        user:"root",
        password:"123456",
        database:"test"
    });
    const sql ='select * from articleDetail where id ='+id;
    console.log(sql);
    db.query(sql,function(err,result){
        if(err) return console.log(err.message);
        if (result.length>0){
            articleDetail=result;
        }
    });
    console.log(articleDetail);
    res.send(articleDetail);

});