Question

I recently started with the Qt Creator and C++ and wanted to use a PostgreSQL database for my needs. So I figured out how to get the driver, included all the tags needed and started executing some querys.

I created new tables, inserted some data, updated them afterwards. It all worked just fine, until I tried to simply select some rows. The query just ended without an error-message and a response of NULL.

The command was a simple SELECT-Command:

query = db.exec("SELECT id FROM users WHERE name = 'Testuser';");

But a basic SELECT doesn’t work either:

query = db.exec("SELECT * FROM users;");

If I now copy exactly this query and put it as a SQL-Statement directly into pgAdmin, it works just fine and responses with the user-id.

I tried quotation marks for the tablename, I tried the full row-names (SELECT users.id FROM public.users WHERE users.name = 'Testuser';) and large and small letters because of some tips from google – nothing worked.

Every query works just fine, but if I try a SELECT, it just always responses with NULL. Although every single of this SELECT-Querys works just fine in pgAdmin.

Has anyone an idea?

The following tags are used:

#include <QDebug>
#include <QtCore/QCoreApplication>
#include <QtSql/QPSQLDriver>
#include <QSqlDatabase>
#include <QSqlQuery>
#include <QCryptographicHash>
#include <QSqlError>

The full code looks like this:

QString response;
QSqlQuery query;
QSqlDatabase db = QSqlDatabase::addDatabase("QPSQL");

db.setHostName("127.0.0.1");
db.setPort(5432);
db.setDatabaseName("postgres");
db.setUserName("postgres");
db.setPassword("password");

if (db.open())
{
    query = db.exec("SELECT id FROM users WHERE name = 'Testuser';");
    response = query.value(0).toString();
}

db.close();

Answer 1

Qt Assisstant says:

After the query is executed, the query is positioned on an invalid
record and must be navigated to a valid record before data values can
be retrieved (for example, using next()).

So, you should do this to get its first record:

query.first();
QString result = return query.record().value(0).toString();

Also you can iterate over its records:

QStringList results;
while (query.next())
{
    QString result = return query.record().value(0).toString();
    results.append(result);
}

And it’s always a good idea to check query execution error status:

bool res = query.exec(...);
if (res == false)
{
    qDebug() << "SQL ERROR: " << query->lastError().text();
}