如何在nodejs中循环查询mysql？ 因为你应该等到查询结果可用否则循环将永远运行

Question

In the following code I am generating a uuid and then making sure if it already doesn't exist but the problem is variable 'pid' is not working that is [pid] written after query is not working somehow and I don't why. now plz don't suggest y I am making sure if uuid already does not exist cuz they are unique all the time, just tell me the fact why js variabel var pid = uuid() is not being accepted by mySql connection.query
FIRST EDIT: The first connection.query is not even getting executed.
SECOND EDIT: After hours of debugging the problem still remains unsolved but now I know the cause which is not javascript variable but the result of the connection.query which is not readily available so the loop continues to run not waiting for the result. After reading articles I somehow know that promises may do my work but idk how to use them so see ya later until I learn them(also I am changing title of the question since error is not with js variable but with the returning of result of mysql connection.query function).
Title changed from:mySQL query is not selecting upon using Javascript variable nodejs
To: above title

//load user model and mysql
const mysql = require('mysql')
const configDB = require('../config/configDB')
const connection = mysql.createConnection(configDB.connection)
//load uuidv4 to generate user ids
const uuid = require('uuid')
module.exports = function(done){
    var exist = true
    do{
        const pid = uuid.v4()
        connection.query("SELECT * FROM PATIENT WHERE PATIENT_ID = ?",[pid], function(err, prows){ 
//query on the above line is not even getting executed and it is because of [pid] but don't know y
                if(err)
                    return done(err)
                if(prows.length){
                   exist = true
                } else {
                    connection.query("SELECT * FROM DOCTOR WHERE DOCTOR_ID = ?",[pid], function(err, drows){
                        if(err)
                            return done(err)
                        if(drows.length){
                            exist = true
                        } else {
                            connection.query("SELECT * FROM ADMIN WHERE ADMIN_ID = ?",[pid], function(err, arows){
                                if(err)
                                    return done(err)
                                if(arows.length){ //corrected from drows to arows after someone answered but the problem is something else and it is still unsolved
                                    exist = true
                                } else {
                                    exist = false
                                    return pid
                                }
                            })
                        }
                    })
                }
            })
    
    
        }while(exist)
    }

Answer 1

In this line of your code

if(drows.length){
    exist = true
}

Seems it should be

if(arows.length){ //it should be arows.length because arrows.length is never used
    exist = true
}

Answer 2

Earlier my loop was keep generating new pids because all it was doing was generating a new pid then checking if that exist or not but while querying database it was not waiting for the result and since the result was not readily available so the loop was continuing to run till infinity(after debugging via vs code). I used promise to wait for result of loop like this in generateUserId.js file I defined my function as follows

module.exports = generateUserId

async function generateUserId(){
    let exist = true
    do{
        let pid = uuid.v4()
        const id = await uuidExistOrNot(pid)
        if(id===pid){
            exist = false
            return id
        }
    }while(exist)
}

function uuidExistOrNot(pid){
    return new Promise((resolve, reject) => {
        const selectPQuery = "SELECT * FROM PATIENT WHERE PATIENT_ID = ?"
        const selectDQuery = "SELECT * FROM DOCTOR WHERE DOCTOR_ID = ?"
        const selectAQuery = "SELECT * FROM HADMIN WHERE ADMIN_ID = ?"

        connection.query(selectPQuery, [pid], (err, prows)=>{
            if(!prows.length){
                connection.query(selectDQuery, [pid], (err, drows)=>{
                    if(!drows.length){
                        connection.query(selectAQuery, [pid], (err, arows)=>{
                            if(!arows.length){
                                resolve(pid)
                            } else {
                                reject(err)
                            }
                        })
                    } else {
                        reject(err)
                    }
                })
            } else  {
                reject(err)
            }
        })
    })
}

then when to use it I m importing in that file where I wanna use and then use it as:

generateUserId()
.then(function(id){
 //do what u wanna do with id here that is being generated by generateUserId() 
})
.catch(err=>console.log(err))