Store mysql query rows in variable for later use

问题

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.

回答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
}

来源：https://stackoverflow.com/questions/44945202/store-mysql-query-rows-in-variable-for-later-use

标签

mysql

node.js

node-mysql