node mysql sequential query execution - mysql

Question

I am trying to fetch some results and do the further processing based on those results, but I can't proceed to work it sequentially,
var sql = query1;
con.query(sql, function (err, results) {
if (err) throw err;
// ids => 5,2,3,4
for (i = 0; i < results.length; i++) {
target_user = results[i].ID
var sql = "DELETE QUERY";
con.query(sql, function (err) {
if (err) throw err;
console.log(target_user)
var sql = "INSERT QUERY";
console.log(sql)
con.query(sql, function (err) {
if (err) throw err;
})
})
}
})
The above code runs asynchronously, What I expect is an output in a loop like this
// "DELETE QUERY";
//5
// "INSERT QUERY";
// "DELETE QUERY";
//2
// "INSERT QUERY";
and so on..
but what I get is
// "DELETE QUERY";
//5
// "DELETE QUERY";
//5 //not fetching the next array val
// "INSERT QUERY";
// "INSERT QUERY";
Any help is much appriciated.
EDIT
from answers I updated code like this
now the code looks like this
aysnc.forEach(results, function(elem, callback){
target_user = elem.id
console.log('out')
console.log(target_user)
con.query(sql, function (err) {
if (err) throw err;
console.log('in')
console.log(target_user)
})
})
A strange thing happened that output is
out
5
in
5
out
2
in
5 //when it is supposed to be 2

Answer 1

You can still use npm module async in a different way
const mysql = require('mysql');
const async = require('aynsc');
var db; //database variable
async.series([
//creates DB connection and connects
function(callback){
db = mysql.createConnection(DB_INFO); //DB_INFO is an Object with information on the BD to be connected
db.connect(function(err){
if (err) {
console.error('error connecting: ' + err.stack);
return;
}
callback(); //goes to the next function
});
},
//performs the Query 1
function(callback){
db.query('QUERY1', function(){
callback(); //goes to the next function
});
},
//performs the Query 2 only after Query 1 is finished
function(callback){
db.query('QUERY2', function(){
db.end(); //closes connection
callback();
});
}
]);

Answer 2

You could use recursion to solve something like this. Keep calling function until there is no elements left in results
con.query(sql, function (err, results) {
if (err) throw err;
deleteAndInsertResults(results);
})
function deleteAndInsertResult(results)
{
target_user = results[0].ID
var sql = "DELETE QUERY";
con.query(sql, function (err) {
if (err) throw err;
console.log(target_user)
var sql = "INSERT QUERY";
console.log(sql)
con.query(sql, function (err) {
if (err) throw err;
results.shift();
if(results.length){
return deleteAndInsertResult(results);
}
})
})
}

Answer 3

In node.js FOR loop will be executed parallely, so use async module or PROMISE below is an example using async
var async = require('aynsc');
con.query(sql, function (err, results) {
if (err) throw err;
// ids => 5,2,3,4
async.forEach(results, function(elem, callback){
target_user = results[i].ID
var sql = "DELETE QUERY";
con.query(sql, function (err) {
if (err) throw err;
console.log(target_user)
var sql = "INSERT QUERY";
console.log(sql)
con.query(sql, function (err) {
if (err) throw err;
callback()
})
})
}, function(err){
//final callback once loop is done
});
})