Select from SQLite with Qt

Question

I try to deal with SQLite database on Qt 4.5.3 on Linux. I've already created the databsae.

Then, I try to perform select on Qt:

db = QSqlDatabase::addDatabase("QSQLITE");
db.setDatabaseName(filename); // Here is FULL path to the database. I've checked it twice :)
bool ok = db.open();
qDebug() << db.tables();

QSqlQuery query;
query.exec("select * from lessons");
qDebug() << query.size();
qDebug() << query.isSelect();
qDebug() << query.isValid();

But debug console says:

("lessons", "weeklessons", "weeks") 
-1 
true 
false

Why it's select nothing? What I have doing wrong?

It seems that .size() doesn't work with Sqlite :( That is why You have -1. — Sebastian Dusza
– Sebastian Dusza, Commented Mar 23, 2011 at 20:33

CHW · Accepted Answer · 2015-12-02 06:55:25Z

9

The isValid() method returns true if the query is positionned on a valid record, but after calling exec(), it isn't : you have to move to a valid record first, for example with query.first() or query.next(). See Qt docs : http://doc.qt.io/archives/4.6/qsqlquery.html

The size() returning -1 doesn't mean there is no result : SQLite is one of the databases for which the size of the query is not directly available (look in the documentation for QSqlDriver::hasFeature()). You can check that rows are returned and find the size with a loop and query.next().

Depending on what you want to do with the result of your select, you could also use QSqlQueryModel instead of QSqlQuery.

edited Dec 2, 2015 at 6:55

CHW

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answered Apr 16, 2010 at 14:25

Leiaz

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1 Comment

Harald Scheirich Over a year ago

That looks like the issue. @vas3k in general you should check the return value of exec() to see wether the execution of the query succeeded.

matt_man22 · Accepted Answer · 2010-02-23 03:15:13Z

3

In your "QSqlQuery query;" declaration, you have to specify the database connection, e.g. "QSqlQuery query(db)"

answered Feb 23, 2010 at 3:15

matt_man22

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Comments

Ton van den Heuvel · Accepted Answer · 2010-02-23 20:13:06Z

1

Given that your program reports that the query is invalid, have a look at the error message as follows:

QDebug() << query.lastError().text();

This should help you in debugging the problem.

answered Feb 23, 2010 at 20:13

Ton van den Heuvel

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4 Comments

vas3k Over a year ago

Thanx for the answer, yes I know. But it output an empty string (just one space): ("lessons", "weeklessons", "weeks") " " -1

Ton van den Heuvel Over a year ago

Are you sure the query actually works? Try it from the SQLite prompt. Maybe adding a semi-colon at the end of your query might help? Finally, I would try getting the SQLite example that is shipped with Qt to work.

vas3k Over a year ago

Yes it works in my SQLite Viewer: lh3.ggpht.com/_hyIyDtqe5N0/S4KNBpfapeI/AAAAAAAAAU0/3MEliEIy8G4/… Adding semicolon didn't helps too. And where can I find Qt SQLite examples? I found only MySQL.

Ton van den Heuvel Over a year ago

This one for example: qt.nokia.com/doc/4.6/sql-sqlwidgetmapper.html It uses an in memory SQLite database, but it should be easy to rewrite the example to use an SQLite database from disk.

DeDe · Accepted Answer · 2010-08-05 07:26:52Z

0

This can give you a answer: http://www.qtcentre.org/threads/7904-Is-there-any-other-reason-why-QSqlQuery-size%28%29-returns-1

answered Aug 5, 2010 at 7:26

DeDe

1

1 Comment

ypercubeᵀᴹ Over a year ago

Answers here are considered best if they don't only provide a link (however helpful that may be) but also explain the reasons of the problem or describe a solution. You can always edit your answers to improve them.

CHW · Accepted Answer · 2015-12-02 07:01:30Z

0

Without knowing much about Qt, I was asking myself one question: How does the QSqlQuery know which connection to use. Looking that up in the manual revealed that there are basically two possibilities to execute a query:

query = db.exec("select * from lessons");
Using the database connection, and executing from there.
QSqlQuery query( db );
Constructing the query using the existing database and then executing the query:
query.exec("select * from lessons");

edit: After reading a bit more, it seems that default connections are supported, so your example should work…

Could you try passing the query as the constructor's value instead? Maybe it works then.

QSqlQuery query("select * from lessons");

edited Dec 2, 2015 at 7:01

CHW

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answered Feb 22, 2010 at 13:56

poke

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2 Comments

vas3k Over a year ago

Qt makes it as "default connection". Yes, of course I tried to do this way. Nope, same errors. db = QSqlDatabase::addDatabase("QSQLITE"); db.setDatabaseName(filename); bool ok = db.open(); qDebug() << db.tables(); QSqlQuery query = db.exec("select * from lessons"); qDebug() << query.size(); output: ("lessons", "weeklessons", "weeks") -1

vas3k Over a year ago

And I tried to add query to constructor's value. Same errors, don't work too. Very strange :(

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