First use addDatabase like this :
Qt Code: 
  1. bool myDatabase::createDatabaseConn()
  2. {
  3.     QSettings settings("ATSTech", "ats_shopfront");
  4.  
  5.     settings.beginGroup("database");
  6.  
  7.     QSqlDatabase db = QSqlDatabase::database();
  8.     if( db.isValid() )
  9.         return true;
  10.  
  11.     db = QSqlDatabase::addDatabase("QMYSQL");
  12.  
  13.     db.setHostName(settings.value("server").toString());
  14.     db.setDatabaseName("dbname");
  15.     db.setUserName(settings.value("databaseUsername").toString());
  16.     db.setPassword(settings.value("databasePassword").toString());
  17.  
  18.     if (!db.open()) {
  19.         //QMessageBox::critical(0, QObject::tr("Database Error"), db.lastError().text());
  20.         return false;
  21.     }
  22.     return true;
  23.  
  24.     settings.endGroup();
  25. }
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