Approach to multiple MySQL queries with Node.js

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One should avoid the pyramid of doom:

var express = require('express');
var Q = require('Q');
var app = express();

app.get("https://stackoverflow.com/",function(req,res){
    var mysql      = require('mysql');

    var connection = mysql.createConnection({
        host     : 'localhost',
        user     : 'root',
        password : ''
    });

    connection.connect();

    function doQuery1(){
        var defered = Q.defer();
        connection.query('SELECT 1 AS solution',defered.makeNodeResolver());
        return defered.promise;
    }

    function doQuery2(){
        var defered = Q.defer();
        connection.query('SELECT 2 AS solution',defered.makeNodeResolver());
        return defered.promise;
    }

    Q.all([doQuery1(),doQuery2()]).then(function(results){
        res.send(JSON.stringify(results[0][0][0].solution+results[1][0][0].solution));
        // Hint : your third query would go here
    });

    connection.end();

});

app.listen(80);
console.log('Listening on port 80');

This sample show a result which depend of 2 independent computed values.
Each of these values a queried in doQuery1 and doQuery2. They are executed in sequence, but asynchronously.

Next you can see Q.all(... which basically call the “then” callback on success. Within that callback, the calculation is done.

Using promises (details : Github Q: promise for Javascript and wikipedia ) permit to make your code cleaner, separate computation and handling of results and move things arround.

Look at how easy it would be to add “doQuery3” as prerequisit for your calculation !

And bellow the “package.json” bellonging to the sample code:

{
    "name": "hello-world",
    "description": "hello world test app",
    "version": "0.0.1",
    "private": true,
    "dependencies": {
        "express": "3.2.0",
        "q": "0.9.3",
        "mysql":"2.0.0-alpha7"
    }
}

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