[mapserver-users] About FILTER and FILTERITEM

[Rahkonen Jukka (Tike)]

Hi,

I started to play with filter and filteritem  because I happened to see this question http://gis.stackexchange.com/questions/65724/mapserver-6-2-concatenate-attributes-in-filter. I thought that the idea was odd and unpractical but interesting and I thought it would be fast to test. It was not but at least it was educational.

I can do such concatenation simply with Spatialite data by using the layer DATA as
CONNECTION "\ms4w\apps\states_spl_3.sqlite"
DATA "select * from states_view where STATE_NAME||STATE_FIPS='Illinois17'"

I am ready to believe that such concatenation with FILTER is not possible with the 6.x series. Other opinions?

-Jukka Rahkonen-

Lime, Steve D (MNIT) wrote:
> 
> FIlters are used primarily under the hood for certain types of query operations.
> The main differences are:
> 
>   - FILTERs are defined in the native query language of the underlying driver. So
> for PostGIS you'd actually write a little SQL snippet.
>   - FILTERs are applied when data are pulled from the underlying data source (like
> a WHERE clause) and have no impact on rendering choices.
> 
> For drivers that don't have an native query capability (e.g. shapefiles) the syntax
> is identical to expressions. For drivers that do have a native query capability
> there really isn't much use for filters since you can just add a where clause to the
> data statement.
> 
> Where I have found them useful sometimes is for simplifying class expressions.
> Let's say you have a shapefile and you only want to show features of a certain
> type, and then for those features you want to draw things differently based on
> another attribute.
> 
> You could use logical expressions:
> 
> CLASS
>   EXPRESSION ([type] = 1 AND '[attr]' = 'a')
>   ...
> END
> CLASS
>   EXPRESSION ([type] = 1 AND '[attr]' = 'b')
>   ...
> END
> 
> or you could use a combination of FILTER and EXPRESSIONS
> 
> FILTERITEM 'type'
> FILTER '1'
> 
> CLASSITEM 'attr'
> CLASS
>   EXPRESSION 'a'
>   ...
> END
> CLASS
>   EXPRESSION 'b'
>   ...
> END
> 
> Personal preference I guess and the second option is a bit more performant,
> especially in older versions. I guess my recommendation would be avoid FILTERs
> unless you have a good reason to use them.
> 
> Steve
> 
> ________________________________________
> From: mapserver-users-bounces at lists.osgeo.org [mapserver-users-
> bounces at lists.osgeo.org] on behalf of Rahkonen Jukka  (Tike)
> [jukka.rahkonen at mmmtike.fi]
> Sent: Wednesday, May 14, 2014 8:05 AM
> To: mapserver-users at lists.osgeo.org
> Subject: Re: [mapserver-users] About FILTER and FILTERITEM
> 
> Hi,
> 
> Thank you very much, Christy. By your example I made some tests and now
> filters like this work for me FILTER (([WORKERS]>20000000) OR
> ("[STATE_FIPS]"="17"))
> 
> The secret element was the right syntax for string attributes
> ("[STATE_FIPS]"="17").
> Now when I know how it must be used I can find corresponding examples from
> the expression page of Mapserver documentation. Could someone explain
> shortly what is the difference between filter and expression? To me they seem
> to do pretty similar work.
> 
> -Jukka Rahkonen-
> 
> Christy Nieman wrote:
> 
> > You do not need to set both FILTERITEM and FILTER.  You can just set
> > FILTER to an expression using as many attributes as you need. e.g.
> > FILTER (( [POPULATION] < 500000 ) AND ("[DISPLAY]" == "1")). Just like
> > you can have multiple attributes in a class EXPRESSION.
> >
> > I think it's the section of the document that is key - the "MapServer
> Expressions"
> > section talks about using FILTER and EXPRESSION in more complex
> > logical expressions, where as the section you quoted is talking about
> > basic string comparison.
> >
> > Christy
> >
> > On 14/05/14 08:03 AM, Rahkonen Jukka (Tike) wrote:
> > > Hi,
> > >
> > >  From document page: http://mapserver.org/mapfile/expressions.html
> > > "To use a string comparison for filtering a LAYER, both FILTERITEM
> > > and FILTER
> > must be set. FILTERITEM is set to the attribute name. FILTER is set to
> > the value for comparison. The same rule applies to CLASSITEM in the
> > LAYER object and EXPRESSION in the CLASS object."
> > >
> > > That would mean that only one attribute from the data can be used
> > > for
> > building a filter. Is this true or just old information living in the document?
> > >
> > > -Jukka Rahkonen-
> > > _______________________________________________
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> > > mapserver-users at lists.osgeo.org
> > > http://lists.osgeo.org/mailman/listinfo/mapserver-users
> >
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