Qt Sql not able to bind variables to QSqlQuery prepare Statement - c++

Question

I am trying to fetch some records from MySQL database by using a prepared statement by using QSqlQuery as:
QString username=ui->textEdit_password->toPlainText();
QString password=ui->textEdit_password->toPlainText();
QSqlQuery query;
query.prepare("SELECT * FROM login_access WHERE username=? AND password=?");
query.addBindValue(username);
query.addBindValue(password);
query.exec();`
When i run :
std::string q_str1=query.executedQuery().toUtf8().constData();
std::cout<<"Query : "<<q_str1<<"\n";
It outputs : Query : SELECT * FROM login_access WHERE username=? AND password=? where the "?" has not been replaced and the query returns nothing since the "?" character is compared to the database records.
On running the query: SELECT * FROM login_access, the query returns all the database records in the login_access table.
I have also tried replacing the "?" with placeholders ":uname",":pass" and changed query.addBindValue(username); to query.bindValue(":uname",username);, and done same with password field.
I am running QtCreator 4.4.1
Thanks.

Answer 1

Use query.bindValue( ...) because this sets the placeholder value.
I tested executedQuery() on one of my SQL statements with placeholders and it returned a string with just the placeholders, not the values. The documentation does say that in most cases it the same string as lastQuery().
http://doc.qt.io/qt-5/qsqlquery.html#executedQuery
You have confirmed that your SQL statement without the where clause works so the next stage is to check you are binding what you think you are binding. To do this use boundValue(const QString placeholder) to find out if the placehold value is being bound.
It might also be useful to check the query has run OK.
So, after your query.exec you should put the following (assuming these are your placeholders) just to check these things:
qDebug() << query.lasterError();
qDebug() << query.boundValue(":uname");
qDebug() << query.boundValue(":pass");