Qt-interest Archive, July 2004
Retrieving null database query values
Message 1 in thread
After over a year of blissful Qt database development, I am suddenly
having so much trouble that I ripped out my entire Qt installation and
reinstalled thinking it must somehow have corrupted. The only thing I
can think that has changed is a recent upgrade to KDE 3.2.3.
My current application makes extensive use of left joins. I have always
checked for null values with something like:
if ( query.value(0).toString().isNull() ) {
Suddenly, in one spot in my application, this seems to have stopped
working. I stop the application in a debugger right before the query is
made, dump the query string and run it manually against the database and
it returns a null value exactly as expected. The application doesn't
detect the null value.
I then noticed that there is a method, QVariant::isNull() which I had
not noticed before. I thought, perhaps, that is what I should use
instead but that did not work either.
I then returned the value of query.value(0).toString() (which is
supposed to QString::null or so I believe) in a QMessageBox for kicks
and to my great surprise it returned the value "0".
Has something changed in Qt? Am I doing something incorrectly? How does
one properly check for null values?
I am currently working around it by using toUInt() and checking for a
zero returned but something seems seriously awry. Thanks - John
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Message 2 in thread
Yes, there is something weird with QString and null strings. I am on the track
of this a long time ago, but I couldn't get it. Look at this thread where I
expose a similar problem to yours:
http://lists.trolltech.com/qt-interest/2004-06/thread00443-0.html
The correct way to compare if a value is null is with
query.isNull(0)
QString s = ( query.isNull(0) ? QString() : query.value(0) );
Hope this helps!
On Monday 12 July 2004 11:43, John A. Sullivan III wrote:
> After over a year of blissful Qt database development, I am suddenly
> having so much trouble that I ripped out my entire Qt installation and
> reinstalled thinking it must somehow have corrupted. The only thing I
> can think that has changed is a recent upgrade to KDE 3.2.3.
>
> My current application makes extensive use of left joins. I have always
> checked for null values with something like:
>
> if ( query.value(0).toString().isNull() ) {
>
> Suddenly, in one spot in my application, this seems to have stopped
> working. I stop the application in a debugger right before the query is
> made, dump the query string and run it manually against the database and
> it returns a null value exactly as expected. The application doesn't
> detect the null value.
>
> I then noticed that there is a method, QVariant::isNull() which I had
> not noticed before. I thought, perhaps, that is what I should use
> instead but that did not work either.
>
> I then returned the value of query.value(0).toString() (which is
> supposed to QString::null or so I believe) in a QMessageBox for kicks
> and to my great surprise it returned the value "0".
>
> Has something changed in Qt? Am I doing something incorrectly? How does
> one properly check for null values?
>
> I am currently working around it by using toUInt() and checking for a
> zero returned but something seems seriously awry. Thanks - John
--
[ signature omitted ]
Message 3 in thread
Santiago Capel wrote:
> Yes, there is something weird with QString and null strings. I am on the track
> of this a long time ago, but I couldn't get it. Look at this thread where I
> expose a similar problem to yours:
>
> http://lists.trolltech.com/qt-interest/2004-06/thread00443-0.html
>
> The correct way to compare if a value is null is with
>
> query.isNull(0)
>
>
> QString s = ( query.isNull(0) ? QString() : query.value(0) );
>
>
> Hope this helps!
><snip>
Yes, thank you very much. However, I am a little concerned by the
warning in the documentation:
"Note that for some drivers isNull() will not return accurate
information until after an attempt is made to retrieve data"
What does that mean? Is this safe if all I am testing for is the null value?
By the way, how does one cut and paste out of the Qt documentation? I am
viewing it under KDevelop.
Thanks, all - John
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[ signature omitted ]
