query prepare with sql math

This is my code:

Code:

---

QSqlQuery query;
 
		query.prepare("select :a / :b");
 
		query.addBindValue( ui.a_lineEdit->text() );
		query.addBindValue( ui.b_lineEdit->text() );
 
		query.exec();
 
 
		while (query.next()) {
		QString res = query.value(0).toString();
		ui.res_lineEdit->setText(res);

---

ui.a_lineEdit->text() is 9 and ui.b_lineEdit->text() is 3
If I try to +,- or * the result shows up in ui.res_lineEdit->setText(res), but when I try to divide it doesnt show me anything. Why?

Renan

What do query.exec() and query.lastError() return?

code:

Code:

---

QSqlQuery query;
 
		query.prepare("select :a / :b");
 
		query.addBindValue( ui.a_lineEdit->text() );
		query.addBindValue( ui.b_lineEdit->text() );
 
		query.exec();
		QMessageBox::critical(0, tr("Error"), 
		QString("The error:\n%1").arg(query.lastError().text())); 
 
 
		while (query.next()) {
		QString res = query.value(0).toString();
		ui.res_lineEdit->setText(res);

---

query.lastError() returns me nothing.

What could I do to know what query.exec() returns?

Renan

exec() returns true or false. if exec() returns false that means that an error appeared during query executing.

Code:

---

QSqlQuery query;
 
		query.prepare("select :a / :b");
 
		query.addBindValue( ui.a_lineEdit->text() );
		query.addBindValue( ui.b_lineEdit->text() );
 
		if (!query.exec()){
			QMessageBox::critical(0, tr("Error"), 
			QString("There is an error with query.exec()!"));
		}else{
 
			query.exec();
			QMessageBox::critical(0, tr("Error"), 
			QString("There is no error with query.exec():\n%1").arg(query.lastError().text()));		
 
			while (query.next()) {
			QString res = query.value(0).toString();
			ui.res_lineEdit->setText(res);
			}

---

There is no error with query.exec() and query.lastError() returns me nothing.

Any suggestions?

Renan

sorry, it works.

what query.exec() and query.lastError() returns?

this code returns "2"

Code:

---

QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE");
	db.setHostName("localhost");
	db.setDatabaseName("test.db");
	db.setUserName("system");
	db.setPassword("sysdba");
	if (!db.open())
		qWarning() << db.lastError().text();
	else {
		QSqlQuery query(db);
		query.prepare("CREATE TABLE test (a int, b int)");
		if (!query.exec())
			qWarning() << query.lastError().text();
		else {
			qDebug() << query.exec("INSERT INTO test (a, b) VALUES (1, 2)");
			qDebug() << query.exec("INSERT INTO test (a, b) VALUES (3, 4)");
			qDebug() << query.exec("INSERT INTO test (a, b) VALUES (5, 5)");
		query.prepare("SELECT :a / :b");
			query.addBindValue(4);
			query.addBindValue(2);
			qDebug() << query.exec();
			query.next();
			qDebug() << query.value(0);
		}
	}

---

I have tested your code and it returns "2".

Something strange is happening, because when I change

Code:

---

query.prepare("select :a / :b");

---

to

Code:

---

query.prepare("select :a + :b");

---

or

Code:

---

query.prepare("select :a - :b");

---

or

Code:

---

query.prepare("select :a * :b");

---

it returns me the right result.
Do you see anything wrong with my code?


Renan

what kind of database do you use? mysql, right?

yes mysql.
here is the the pushbutton code:

Code:

---

void testeMYSQL2::on_soma_pushButton_clicked()
{
	QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL");
        db.setHostName(ui.hostname_lineEdit->text());
        db.setDatabaseName(ui.database_lineEdit->text());
        db.setUserName(ui.username_lineEdit->text());
        db.setPassword(ui.password_lineEdit->text());
	db.open();
	if (!db.open()){
		QMessageBox::critical(0, tr("Error"), 
		QString("The error:\n%1").arg(db.lastError().text())); 
	}
	else{
		QSqlQuery query;
 
		query.prepare("SELECT :a / :b");
 
// 		query.addBindValue( ui.a_lineEdit->text() );
// 		query.addBindValue( ui.b_lineEdit->text() );
// 		query.addBindValue( 400 );
// 		query.addBindValue( 2 );
 		query.bindValue(":a", ui.a_lineEdit->text() );
 		query.bindValue(":b", ui.b_lineEdit->text() );
 
		if (!query.exec()){
			QMessageBox::critical(0, tr("Error"), 
			QString("There is an error with query.exec()!"));
		}else{
 
//			query.exec();
			QMessageBox::critical(0, tr("Error"), 
			QString("There is NO error with query.exec():\n%1").arg(query.lastError().text()));		
 
			while (query.next()) {
			QString res = query.value(0).toString();
			ui.res_lineEdit->setText(res);
			}
		}
	}
	db.close();
}

---

Renan

Here is something really strange:
if I change:

Code:

---

query.prepare("SELECT :a / :b");

---

to

Code:

---

query.prepare("SELECT (:a / :b)*1");

---

I get the result.
Maybe a QT bug?

Renan

Indeed it behaves strange. Could you prepare a minimal compilable example?

I have uploaded a simple compilable example, as jacek asked.

Renan

I have uploaded it again.

I can't download testemysql2.cpp

I have uploaded it again.

Renan

looks like bug in Qt.
this code works, you can use it

Code:

---

...
query.prepare(QString("select %1/%2").arg(ui.lineEdit->text()).arg(ui.lineEdit_2->text()));
...

---

Thanks again spirit.

I will wait one or two days to report these to trolltech. Maybe someone knows why this is happening.

I'm using QT 4.3.4 open source Linux Ubuntu 8.04. I don't if this happens with QT 4.4.1.

Renan

I've tested on Qt 4.4.1, the result is the same.

I've minimized it a bit further (see attachment) and it appears that for Qt 4.4.1 on Linux I get the "No results." message, which means that the database doesn't return any data. Of course it works for * operator.

I've also found a workaround:

Code:

---

query.addBindValue( 4.0 );
query.addBindValue( 2.0 );

---