[SOLVED]QSqlQuery::value: not positioned on a valid record

bobby

Hello All,

After a few hours of reviewing the developer network I've got to ask: Why am I getting this error? I see a lot of people recommend making sure to call @query.next()@ before @query.value(0).toString()@ but I've called that before.
I've already connected to the db, here I am explicitly calling it to list the result of the query in a text area.

Code is as follows:

@
QSqlDatabase db = QSqlDatabase::database("default_db_connection");
QSqlQuery query("SELECT table_name FROM information_schema.tables WHERE table_schema ='public';", db);

while(query.next());
{
    if(query.isActive())
    {
        QMessageBox::information(0,"Good Query", "Good Query.  It\'s active");
    }
    else
    {
        QMessageBox::warning(0, "Bad Query", "Bad Query, It\'s inactive");
    }
    ui->textEdit->append(query.value(0).toString());

}

@

goetz

check the output of

@
qDebug() << query.lastError();
@

I suspect an SQL syntax error, as you should write:

@
QSqlQuery query("SELECT table_name FROM information_schema.tables WHERE table_schema ='public'", db);
@

Note the missing backslashes and leave out the semikolon at the end.

alexisdm

Remove the semi-colon after the while(query.next()) which makes the loop skip all valid records if there was any.

You should test isActive() only once outside the loop (I don't think there is a way to discriminate between an error and the last record once the query has successfully started) and show the error if that test fails as Volker said :
@if(query.isActive()) {
QMessageBox::information(0,"Good Query", "Good Query. It's active");
while(query.next())
{
ui->textEdit->append(query.value(0).toString());
}
} else {
QMessageBox::warning(0, "Bad Query",
QString("Bad Query, It's inactive: %1").arg(query.lastError().text());
}@

bobby

[quote author="alexisdm" date="1312850811"]Remove the semi-colon after the while(query.next()) which makes the loop skip all valid records if there was any.

You should test isActive() only once outside the loop (I don't think there is a way to discriminate between an error and the last record once the query has successfully started) and show the error if that test fails as Volker said :
@if(query.isActive()) {
QMessageBox::information(0,"Good Query", "Good Query. It's active");
while(query.next())
{
ui->textEdit->append(query.value(0).toString());
}
} else {
QMessageBox::warning(0, "Bad Query",
QString("Bad Query, It's inactive: %1").arg(query.lastError().text());
}@
[/quote]

Awesome! This plus Volker's

[quote author="Volker" date="1312844579"]check the output of

@
qDebug() << query.lastError();
@

I suspect an SQL syntax error, as you should write:

@
QSqlQuery query("SELECT table_name FROM information_schema.tables WHERE table_schema ='public'", db);
@

Note the missing backslashes and leave out the semikolon at the end.[/quote]

Both made it work!

You guys rock. Thank you for taking time to answer!

Harshi

You should call query.first() before you can access returned data. additionally if your query returns more than one row, you should iterate via query.next().

mduzoylum

Hi @bobby,

Afer sql.exec(), you have to add sql.next() to get position of database record.
Have a nice day.