query prepare with sql math

[GuL]

This is my 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);

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);

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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