QSqlQuery qry;

    if(qry.exec("select * from Users where Nick = '"+nick+"' and Password = '"+password+"'" ))
    {
        int count = 0;
        while(qry.next())
        {
            count++;
        }
        if(count == 1)
        {

            if((nick.toStdString()) == "admin")
            {
                  MainWindow mainWindow;
                user = &admin;
                mainWindow.show();
            }

            if((nick.toStdString()) == student.nick)
                user = &student;

            if((nick.toStdString()) == professor.nick)
                user = &professor;

        }
        if(count < 1)
        {
             ui->errorLabel->setText("Wrong login or password");
        }
    }