websockets and database updates (push on change)

So my question: How can I achieve to get real-time data changes from a table of the database or from a certain query...

There is a module to watch for MySQL events: mysql-events

Official Example :

var MySQLEvents = require('mysql-events');var dsn = {  host:     _dbhostname_,  user:     _dbusername_,  password: _dbpassword_,};var mysqlEventWatcher = MySQLEvents(dsn);var watcher =mysqlEventWatcher.add(  'myDB.table.field.value',  function (oldRow, newRow, event) {     //row inserted     if (oldRow === null) {      //insert code goes here     }     //row deleted     if (newRow === null) {      //delete code goes here     }     //row updated     if (oldRow !== null && newRow !== null) {      //update code goes here     }    //detailed event information     //console.log(event)   },   'match this string or regex');

... which will send the update to all clients, that are connected in real-time, without long polling?

You can use socket.io and prevent the initial http polling entirely by doing this on the client:

var socket = io({transports: ['websocket'], upgrade: false});

To avoid clients from using polling, add this line to the server:

io.set('transports', ['websocket']);

and to send the events to all connected (socket.io) clients from mysql-event use the following :

io.sockets.emit('db-event',eventDataObj)