---- headers.h -----
#include <QtSql>
#include <QSqlDatabase>

#include <QCryptographicHash>
#include <QByteArray>

QSqlDatabase db =QSqlDatabase::addDatabase("QMYSQL");

void dbconnect()
{

	//QSqlDatabase db = QSqlDatabase::addDatabase("QODBC");
	db.setHostName("xxx.xxx.xxx");
	db.setDatabaseName("xxxxx");

	db.setUserName("db_user");
	db.setPassword("xxxxx");



}

void closeDB()
{
	QSqlDatabase::removeDatabase("QMYSQL");
}


----------------------------
login.cpp
-----

#include "headers.h"
#include "logindialog.h"
#include <QtGui>


loginDialog::loginDialog(QWidget *parent)
	: QWidget(parent)
{
	ui.setupUi(this);
}

loginDialog::~loginDialog()
{

}

void loginDialog::showMainWindow()
{



}

void loginDialog::tryLogin()
{
	dbconnect();


	QString uname = ui.username->text();
	QString passwd = ui.passwd->text();

	QCryptographicHash *pwhash = new QCryptographicHash(QCryptographicHash::Md5);

	QByteArray passwdstr = ui.passwd->text().toStdString().c_str();

	pwhash->addData(passwdstr);

	QByteArray string1 = pwhash->result();
	//QMessageBox::information(this, "testi", uname + " " + string1);

	if(db.open())
	{
		QSqlQuery query("Select username from admin WHERE username='"+uname+"' AND password='"+string1+"'");


		query.exec();

		while(query.next())
		{
			QMessageBox::information(this, "Testi", query.value(0).toString());
		}
		qDebug() << query.lastError();
	} else {
		qDebug() << "Error";
	}
}