QSqlQuery::size() not returning actual number?

davecotter

i have execute this select statement:

"SELECT * FROM sqlite_master WHERE type = 'table' AND name = '[tablename]'"

then i ask how many rows i got back (i want to know if this table exists)
but size() returns -1.
here's my workaround, which works, but WHY?

int	Result::CountRows()
{
	int	resultI(0);

	if (IsOk()) {

		if (i_qquery.isSelect()) {
		
			if (i_qquery.isActive()) {
				int	sizeI = i_qquery.size();
				
				if (sizeI >= 0) {
					resultI = sizeI;
					
				} else {
					i_qquery.last();
			
					sizeI = i_qquery.at();
					
					if (sizeI >= 0) {
						resultI = sizeI + 1;
					}
				}

			} else {
				CF_ASSERT(0);
			}
		} else {
			resultI = i_qquery.numRowsAffected();
		}
	}

	return resultI;
}

davecotter

see @JonB 's answer above! thanks!

SGaist

Hi,

I don't remember if it's still the case for SQLite but IIRC not all database engines support returning the query size. You should check that before going further.

JonB

@davecotter
As @SGaist has just posted, size() returns -1 if the driver does not support returning the count of the result set.

Assuming that's the case,

then i ask how many rows i got back (i want to know if this table exists)

"SELECT COUNT(*) FROM sqlite_master WHERE type = 'table' AND name = '[tablename]'"

will just return one row containing the number you want (I would assume either 0 or 1 from your query), you might prefer to use that?

SGaist

You can use QSqlDriver::hasFeature to check for that.

davecotter

@JonB I’m not sure I understand your reply, I already am using that select statement. My q is: what is the proper way to ask how many rows were returned?

Kent-Dorfman

As Gaist mentioned, getting the number of rows from a query response is a very haphazard process when you try to implement it for any ole generic database. MANY DBC backends cache returned rows in manageable groups and use something called a cursor to traverse through them. It is possible, and probable, that the real number of rows returned is far more than is in the currently cached response.

I recommend you used the count() SQL aggregate function to find out how many rows would be returned from your query. It is always guaranteed to return a 1 row result. Looks like JonB also recommended this.

Worst case is that you have to do two queries. First one to find out the size of your result set.

davecotter

@Kent-Dorfman sorry if I wasn’t clear at the start, this is only SQLite and will never change

Kent-Dorfman

@davecotter said in QSqlQuery::size() not returning actual number?:

@Kent-Dorfman sorry if I wasn’t clear at the start, this is only SQLite and will never change

doesn't matter...using the count() method is the more universal way of doing it.

davecotter

@Kent-Dorfman I’m not at all versed at this: is there example code showing how to use count()?

Kent-Dorfman

yes, plenty, but I have no resources off the top of my head. look at any Qt SQL examples you can find. The key thing is that your quere should be:

select count(*) as cnt from sqlite_master WHERE type = 'table' AND name = '[tablename]'

and grab the cnt field from the response

davecotter

@Kent-Dorfman I’m trying to understand “grab the field from the response” ...

Kent-Dorfman

a single field row is returned (called cnt). look at the examples and find one where you get the value of a returned field. there are a few ways to do this.

Executing your QSqlQuery will return a QResultSet object. The field can be accessed using boundValue(0) method of QResultSet. Ya gotta do some googling and research if you want any more detail than that.

mrjj

hi
One way could be. ( note my table is called person. change to yours)

QSqlQuery query("select count(*) as cnt from sqlite_master where type='table' and name='person'");
if (query.next())
    qDebug() << query.value(0).toInt();

However, in org post you said (i want to know if this table exists)
so just as a note
using SQLite version 3.3+
you can say
"create table if not exists TableName ..:"
if you want to check if table exits and create it if not.
Might not be your use case, but its good to know.

davecotter

sorry, i guess i should have made my question more generic than "how many rows will be returned from this specific statement". what i should have asked is:

if i've already performed some statement, and i only have access to the QSqlQuery at this point and no longer know what the actual statement was, how do i ask how many rows that returns? I want it to work for any statement, not just one that asks if a table exists. is my solution just grossly inefficient? is there a better way than this:

  			i_qquery.last();
  	
  			sizeI = i_qquery.at();
  			
  			if (sizeI >= 0) {
  				resultI = sizeI + 1;
  			}

Christian Ehrlicher

As @SGaist already mentioned this only works (and unrelated from Qt) when the underlying database supports this. You can check it with QSqlDriver::hasFeature ...

davecotter

yes we have determined that "size()" doesn't work on SQLite, i understand that.

my question has evolved into: is the way i'm doing it now a bad / terribly inefficient way to do it?
for reference, this is what i'm now doing:

i_qquery.last();
		
sizeI = i_qquery.at();
				
if (sizeI >= 0) {
	resultI = sizeI + 1;
}

thanks

Christian Ehrlicher

It's not very efficient but when you really need the result size for whatever reason you've no other chance.

Kent-Dorfman

@davecotter said in QSqlQuery::size() not returning actual number?:

yes we have determined that "size()" doesn't work on SQLite, i understand that.

my question has evolved into: is the way i'm doing it now a bad / terribly inefficient way to do it?

That's already been answered as well...

JonB

@davecotter said in QSqlQuery::size() not returning actual number?:

@JonB I’m not sure I understand your reply, I already am using that select statement. My q is: what is the proper way to ask how many rows were returned?

No, you must look at what I wrote: it's SELECT COUNT(*) instead of SELECT *. And you retrieve that single result number as the first column in the one & only row returned as the result set, and you must know how to read stuff out of the result set else you couldn't be using queries for much.

Let's just summarise something else:

if i've already performed some statement, and i only have access to the QSqlQuery at this point and no longer know what the actual statement was, how do i ask how many rows that returns? I want it to work for any statement, not just one that asks if a table exists.

Yes, if you have previously executed an arbitrary query and now wish to know how many rows it returned you have no choice but to use your method.

is my solution just grossly inefficient? is there a better way than this:

Well, yes, it is inefficient (though unavoidable if you want the apples for other purposes too). Think of this: the SQL server has a bunch of apples. You ask the server to send you all the apples, and you sit & count how many have arrived to get the answer. (It's not the counting per se which is slow, it's the having to send & receive all the apples in order to count them which is bad.) It's not perfect, is it? Let's hope it has not sent you a lot of apples to count!

A much more efficient way is to ask the server to do the counting at its end, not send all the apples, but send a single number saying how many there are. That is what SELECT COUNT(...) does. It's great for getting a quick total of the apples, but quite useless if you want to look at each one for some other reason.

davecotter

see @JonB 's answer above! thanks!