QSqlQuery and QLineEdit

[spirit]

at first, you have only one column (according to your query text)
second, you set position only on first record (query.next())
and then try to read data from non-existed columns (you have only one column!)
so, try this code

....
     while (query.next()) {
         QString data = query.value(0).toString();//first column == nome_prod
         ....
     }
     ....

....
     while (query.next()) {
         QString data = query.value(0).toString();//first column == nome_prod
         ....
     }
     ....

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read for detailes QSqlQuery::next