I have this SLOT:
Qt Code: 
  1. void ConfSetup::setNextPage()
  2. {
  3.     int currentIndex = ui->stackedWidget->currentIndex();
  4.  
  5.     switch(currentIndex)
  6.      {
  7.        case 1:
  8.         {
  9.              ui->stackedWidget->setCurrentIndex(currentIndex + 1);
  10.         }
  11.        break;
  12.  
  13.        case 2:
  14.         {
  15.            db = QSqlDatabase::addDatabase("QMYSQL");
  16.            db.setDatabaseName("mysql");
  17.            db.setHostName(ui->serverEdit->text());
  18.            db.setPort(ui->portEdit->text().toInt());
  19.            db.setUserName(ui->userEdit->text());
  20.            db.setPassword(ui->passwordEdit->text());
  21.  
  22.            if(!db.open())
  23.            {
  24.             QMessageBox::critical(0, trUtf8("Fail to login"), trUtf8("Wrong user or password"));
  25.  
  26.            }
  27.            else
  28.            {
  29.              ui->stackedWidget->setCurrentIndex(currentIndex + 1);
  30.            }
  31.         }
  32.        break;
  33.  
  34.        case 3:
  35.         {
  36.            QSqlQuery query;
  37.            query.prepare("SELECT user FROM user WHERE user=:user");
  38. //           query.prepare("SELECT user FROM user WHERE user='root'");
  39.            query.bindValue(":user", ui->userDbEdit->text());
  40.            query.exec();
  41.  
  42.            if(query.numRowsAffected() > 0)
  43.             QMessageBox::critical(0, trUtf8("User exist"), trUtf8("This user already token"));
  44.  
  45.            else
  46.            {
  47.              qDebug("%s" , query.executedQuery().toStdString().c_str());
  48.              qDebug() << db.databaseName();
  49.              qDebug() << db.isOpen();
  50.              qDebug() << query.lastError();
  51.              qDebug("%s" , query.lastQuery().toStdString().c_str());
  52.              qDebug("%d" , query.isActive());
  53.              qDebug("%s" , db.driverName().toStdString().c_str());
  54.              qDebug() << db.lastError();
  55.              ui->stackedWidget->setCurrentIndex(currentIndex + 1);
  56.            }
  57.         }
  58.        break;
  59.  
  60.        case 4:
  61.         {
  62.            QSqlQuery query;
  63.            query.prepare("SELECT SCHEMA_NAME FROM INFORMATION_SCHEMA.SCHEMATA WHERE SCHEMA_NAME=:database");
  64.            query.bindValue(":database", ui->dbNameEdit->text());
  65.            query.exec();
  66.  
  67.            if(query.numRowsAffected() == 1)
  68.             QMessageBox::critical(0, trUtf8("Fail to create database"), trUtf8("This database already exist"));
  69.  
  70.            else
  71.              ui->stackedWidget->setCurrentIndex(currentIndex + 1);
  72.         }
  73.        break;
  74.  
  75.        case 5:
  76.         {
  77.            QSqlQuery query;
  78.            query.prepare("INSERT INTO Users(User, Password)" \
  79.                                    "VALUES(:user, :password)");
  80.            query.bindValue(":user", ui->userEdit->text());
  81.            query.bindValue(":password", ui->passwordEdit->text());
  82.            query.exec();
  83.            ui->stackedWidget->setCurrentIndex(currentIndex + 1);
  84.         }
  85.        break;
  86.  
  87.        }
  88.   //  ui->comboBox->addItem(ui->userDbEdit->text());
  89.   //  ui->lineEdit->setText(addSlashes(ui->slashesEdit->text()));
  90. }
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And the connection like so:
Qt Code: 
  1. connect(ui->nextButton0, SIGNAL(clicked()), this, SLOT(setNextPage()));
  2. connect(ui->nextButton1, SIGNAL(clicked()), this, SLOT(setNextPage()));
  3. //etc
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The query is correct because I test it in phpMyAdmin, qDebug output show this:
Qt Code: 
  1. SELECT user FROM user WHERE user=? //should be ... user = user, but formated like this because MySQL doesn't support native prepared statment
  2. "mysql" 
  3. true 
  4. QSqlError(-1, "", "") 
  5. SELECT user FROM user WHERE user=:user
  6. 1
  7. QMYSQL
  8. QSqlError(-1, "", "")
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Even if I execute this query:
Qt Code: 
  1. query.prepare("SELECT user FROM user WHERE user='root'");
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query still return -1.