Store mysql query rows in variable for later use - mysql

Question

I'm doing a monitoring system project in which I have Arduino sensors data being sent to a node.js server (thru GET requests) and then stored in a MySQL DB.
Whenvever I successfully send data to the server, it connects to the MySQL DB and queries the last 5 received records to do some processing.
Therefore, I need to store the rows of those 5 records in a variable for later use. Meaning that I have to get rows from a connection.query in a variable.
I read that the fact that I'm not able to do this is because node.js being async. So my questions are:
Is it possible to do the described tasks the way I'm trying?
If not, is there any other way to do so?
I'm not putting the whole code here but I'm running a separated test that also doesn't run properly. Here it is:
var mysql = require('mysql');
var con = mysql.createConnection({
host : "127.0.0.1",
user : "root",
password: "xxxx",
database: "mydb",
port : 3306
});
var queryString = "SELECT id, temp1, temp2, temp3, temp4, level_ice_bank, flow FROM tempdata ORDER BY id DESC LIMIT 5";
con.connect(function(err) {
if (err) throw err;
});
var result_arr = [];
function setValue (value) {
result_arr = value;
}
con.query(queryString, function (err, rows, fields) {
if (err) throw err;
else {
//console.log(rows);
setValue(rows);
}
});
console.log(result_arr);
It logs:
[]
But if I uncomment console.log(rows); it logs what I need to store in the variable result_arr.
Thanks in advance to all.

Answer 1

You're seeing this behaviour because con.query(...) is an asynchronous function. That means that:
console.log(result_arr);
Runs before:
con.query(queryString, function (err, rows, fields) {
if (err) throw err;
else {
//console.log(rows);
setValue(rows);
}
});
(Specifically, the setValue(rows) call)
To fix this in your example, you can just do:
con.query(queryString, function (err, rows, fields) {
if (err) throw err;
else {
setValue(rows);
console.log(result_arr);
}
});
If you want to do more than just log the data, then you can call a function which depends on result_arr from the con.query callback, like this:
con.query(queryString, function (err, rows, fields) {
if (err) throw err;
else {
setValue(rows);
doCleverStuffWithData();
}
});
function doCleverStuffWithData() {
// Do something with result_arr
}