如何动态更改 qlabel 中的文本以通过按钮获取数据库的下一个值？

Question

I want to change the text in my label by the next value in the database from a pushbutton. I tried the following code, it didn't work-

void category::on_pushButton_clicked()
{
QString d;
QSqlQuery qry;
QVariant b=0,s=0;
b = qry.exec("select count id from category");
if(b>=s)
{
    s++;
    qry.prepare("select heading from category");
    qry.exec();
    d=qry.value(0).toString();
    ui->label_2->setText(d);

}}

It shows the error:-"invalid operands to binary expression('QVariant' and 'QVariant')" in the if statement- error picture(for more understanding)

I changed QVariant to int, and it didn't show any error but when i run it doesn't change the text on clicking pushbutton

Answer 1

Firstly, you should check it in this way: "if (qry.exec())". Secondly, you can see what's wrong with this "QString log = qry.lastError().text();"