QSqlQuery::lastquery() returns blank

I hav configued the Mysql successfully, now its showing QMYSQL in availabel drivers list.

I configured Mysql Server5.0 on my local machine and created database.

Using below code ,it connects to database but when I query it ,doesnt return any records,
so I tried checking my query with Query.lastquery() just before I execute it,and found that its returning blank.that means the query is not set properly ,what could be the error?

Code:

---

QSqlDatabase  db = QSqlDatabase::addDatabase("QMYSQL");
 QSqlQuery query;
 bool ok;
 
  db.setHostName("localhost");
  db.setDatabaseName("ams");
  db.setUserName("root");
  db.setPassword("gauranga");
  db.setPort(3306);
 
   if(!db.isOpen())
        ok = db.open();
 
    if(!ok)
    {
        out << "\nError Driver text: " << db.lastError().driverText() ;
        out << "\nError Database text :" << db.lastError().databaseText();
 
    }
    else
    {
        out <<"DB open successfully";
        ok = query.prepare("SELECT * FROM info");
        out << "\nQuery is: "  << query.lastQuery();
    }

---

If I supply query while creating qsqlquery object then it runs perfectly well and returns resultset.

QSqlQuery query("SELECT * FROM temp",db);


but if I use

ok = query.prepare("SELECT * FROM temp");

then it does not set the query and returns empty string as a result of query.lastQuery()


strange behaviour..