QTSql Read row by row

[A Former User]

I have a problem with following code:

QSqlQuery query(db);
QSqlRecord row = query.record();
if (!query.exec("SELECT member FROM Database"))
	qDebug() << "Error occurred while accessing table: " << query.lastError().text();
if (row.count() > 0) {
	for (int i = 0; i < row.count(); ++i) {
		QString row_str = row.field(i).value().toString();
		cout << i+1 << ". " << row_str.toStdString();
	}
}
else
	cout << "There are no members!\n";

The member column contains 10 random names, but it prints one empty row. db is a mysql driver with opened connection to a database. Thanks for help.

[mranger90]

I've found row.count() to be notoriously unreliable. I believe it's mentioned in the docs somewhere but I don't have a link handy.
You should traverse by using query.next().

[A Former User]

Could you give me an example? How to do that for any number of rows? I have no experience with QtSql and I can't figure it out.
Edit:

while (query.isValid()) {
	QString row_str = query.nextResult();
	cout << row_str.toStdString();
}

I have written this code, but it does not work.

[mrjj]

@ranger281
Hi

   QSqlQuery query("SELECT country FROM artist");
    while (query.next()) {
        QString country = query.value(0).toString();
        doSomething(country);
    }

http://doc.qt.io/qt-5/qsqlquery.html

[A Former User]

How can I load the driver first and then execute the query?

        QSqlQuery query(db);
	query = QSqlQuery("SELECT member FROM Database");
	while (query.next()) {
	        QString row = query.value(0).toString();
		cout << row.toStdString();
	}

This is not working

[mrjj]

@ranger281

you dont need to give it db as paramter.

just open the database somewhere and u can use
QSqlQuery in whole program.

[A Former User]

In that case I get this error:

QSqlQuery::exec: database not open

db is already opened (i added db.open() before creating query).

[JonB]

@ranger281
Where did you do @mrjj's "just open the database somewhere"?
You've edited your post --- now show the code where you open the database and how it relates to where you execute the query. [BTW, did you check it actually opened correctly? QSqlDatabase::open() returns a value...]

[A Former User]

@JonB I opened db in other function called from main and tried doing the same in this code, but it does not work.

db.open();
QSqlQuery query("SELECT member FROM Database");
while (query.next()) {
	QString row = query.value(0).toString();
	cout << row.toStdString();
}

[jsulm]

@ranger281 What does http://doc.qt.io/qt-5/qsqldatabase.html#open return?

[A Former User]

@jsulm This code returns 2:

if (db.isOpen())
	return 2;

I will add the QSqlDatabase::open() in a moment.
Edit:
This code:

	if (db.open())
	        cout << "opened\n";
	QSqlQuery query("SELECT member FROM Database");
	while (query.next()) {
		QString row = query.value(0).toString();
		cout << row.toStdString();
	}

Returns:

opened
QSqlQuery::exec: database not open

When I open the db only in the other function it returns the same error.

[SGaist]

Hi,

You might want to consider printing the error if the open call failed rather than printing that message if it succeed.

Also, where did you setup your database ? Can you show the related code ?

[A Former User]

        QSqlDatabase db;
	db = QSqlDatabase::addDatabase("QMYSQL", "SQLFileDatabase");
	db.setHostName(QString::fromStdString(host));
	db.setDatabaseName(QString::fromStdString(name));
	db.setUserName(QString::fromStdString(user));
	db.setPassword(QString::fromStdString(password));
	db.open();

[SGaist]

Then check whether the open call is successful here and print the error if it's not the case.

Also:

• What version of Qt are you using ?
• What platform ?
• Did you install the MySQL client libraries ?

[JonB]

This post is deleted!

[Gerd]

Change
db = QSqlDatabase::addDatabase("QMYSQL", "SQLFileDatabase");

to

db = QSqlDatabase::addDatabase("QMYSQL");

and everything should work.
An explanation for using a default database can be found in the desricption of QSqlDatabase::addDatabase

[A Former User]

The @Gerd 's solution works. Reading row by row too.
Thank you all for help.