Question

I tried to create a SQLite database with Qt and and I did!! I called my db “prova_db” and it contains the following table:

marker_db

id     site (columns)
0      www.google.it
1      www.youtube.it
2      www.facebook.it

Then, I tried to query my prova_db. Here is code:

int main () {
    QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE");
    db.setDatabaseName("prova_db");
    if (!db.open()) { printf("DB doesn't exist\n");}
    else  {
        QSqlQuery query1;
            query1.exec( "SELECT site FROM marker_db WHERE id = 1");

        int i = query1.numRowsAffected();
        printf("result row: %d\n", i);
        while(query1.next()){
            QString str = query1.value(0).toString();
            printf("result: %s\n", str);
        }
    }
    db.close();
return 0;
}

The result is:

result row : 0
result : (strange char)

instead the result should be:

result row: 1
result : www.youtube.it

where am I doing wrong??

Thank you!

Answer 1

I think you have the wrong expectation: the result of numRowsAffected() tells you how many rows have been altered by the query. Your query does not alter anything, so the result should be either 0 or undefined. Thus, you shouldn’t expect the first output to be risultato riga: 1. Use the size() method instead to find out how many rows have matched your SELECT query.

The second problem is that you are passing a QString object to printf(), which expects a pointer to a null-terminated array of char values instead (when you use the %s format specifier). You need to convert the QString object into a pointer to a C array of characters.