[GRASS-user] Multiple to one question

[Adam Dershowitz, Ph.D., P.E.]

On Apr 15, 2009, at 4:29 PM, Moritz Lennert wrote:

> On 16/04/09 01:00, Adam Dershowitz, Ph.D., P.E. wrote:
>> This is somewhat a followup to my prior post and somewhat a new,  
>> but related problem.
>> I ended up doing what I had proposed, which is I have a table that  
>> has each sample value and location:
>> Location    Value cat
>> a    3.1    1
>> b    2.1    2
>> a    1.1    3
>> c    4.1    4
>> etc.
>> Then I have linked each vector point to multiple categories by using
>> v.edit map=foo layer=1 where="cat==1" type=point tool=catadd cat=3   
>> (for example)  So then I have an vector point that has a cat of  
>> both 1 and 3.  So far so good.
>> If I then try to display Label vectors using Value for the  
>> attribute, it works fine.  Each value is displayed separated by a  
>> slash "/".
>> The problem is if I try to use a SQL query, it is not working as  
>> expected.  I think that what it is doing is just applying the test  
>> to the first value, then showing or hiding the other values based  
>> on that first test (although I am not 100% sure that is what it is  
>> doing).
>> So, for example, if I were to do Use SQL query: Value > 2.0 it  
>> would draw a label like this:  3.1/1.1  And if I were to use a  
>> query of Value < 2.0 it would not display anything.  What I want it  
>> to do is to show all values that are actually greater than 2. and  
>> not to display the others.
>> Any suggestions of what it is doing, or what I can do about it?
>
> Create a view in your database containing the results of your query,  
> including cat values, and link your map to that view, instead of the  
> table. If you want to change the content, just drop the view and  
> recreate it with a different query. You can do all this either  
> directly in sqlite, or via the db.execute GRASS module.
>
> Moritz

I really liked the suggestion, so I tried it....but it doesn't work.
To continue my simple example, I did  create view less2 cat,Value from  
data where Value < 2;
So I end up with a view, as expected.
The problem is that if I now link my map to that view, I get an  
error:  No attribute found for cat 1 (since the object still tries to  
link to 1).  And, the same for 2 and 4.  And, I still see the points  
and slashes (ie it still draws point a and puts a null value then a  
slash then 1.1.
So is there a way to have it not draw a symbol and not draw "null  
text" when it can't find an attribute?  Since, the whole point of what  
I am trying to do in this case is to eliminate certain data with a  
query?

Thanks,

--Adam