.. $Id: querying-HOWTO.txt 6846 2007-09-05 22:53:35Z hobu $

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*****************************************************************************
 Mapscript Query HOWTO
*****************************************************************************

:Author: Sean Gillies
:Contact: sgillies@frii.com
:Revision: $Revision: 6846 $
:Date: $Date: 2007-09-05 15:53:35 -0700 (Wed, 05 Sep 2007) $

..  The next heading encountered becomes our H2
..

.. sectnum::

.. contents::
    :depth: 2
    :backlinks: top


Introduction
============

All examples in this document refer to the mapfile and testing layers
distributed with MapServer 4.2+ and found under mapserver/tests.

Pseudocode
----------

All examples will use a pseudocode that is consistent with the language
independent API reference.  Each line is a statement. For object attributes
and methods we use the dot, '.', operator.  Creation and deletion of objects
will be indicated by 'new' and 'del' keywords.  Other than that, the 
pseudocode looks a lot like Python.

Querying Overview
=================

The Query Result Set
--------------------

Map layers can be queried to select features using spatial query methods
or the attribute query method.  Ignoring for the moment whether we are 
executing a spatial or attribute query, results are obtained like so::

    layer.query()                                 # not an actual method!
    results = layer.getResults()

In the case of a failed query or query with zero results, 'getResults'
returns NULL.

Query Template
--------------

A query will not set results for a layer with a NULL 'template' attribute.
This is an artifact of the querying functionality of the mapserv CGI program
which, unfortunately, lingers in the scripting interface.  I don't like it
either, and would have removed this requirement except for the fact that
some users have scripts that are dependent on this artifact.

Result Set Members
------------------

Individual members of the query results are obtained like::

    ...                                           # continued
    
    if results:
        for i in range(results.numresults):       # iterate over results
            result = results.getResult(i)

This result object is a handle, of sorts, for a feature of the layer,
having 'shapeindex' and 'tileindex' attributes that can be used as
arguments to 'getFeature'.

Resulting Features
-------------------

The previous example code can now be extended to the case of obtaining all
queried features::

    layer.query()
    results = layer.getResults()
    
    if results:
        
        # open layer in preparation of reading shapes
        layer.open()

        for i in range(results.numresults):
            result = results.getResult(i)
            feature = layer.getFeature(result.shapeindex, result.tileindex)
            
            ...  # do something with this feature

        # Close when done
        layer.close()

Backwards Compatibility
-----------------------

Scripts using the 4.2 API can continue to access query result members
through layer methods::

   for i in range(layer.getNumResults()):
       result = layer.getResult(i)
       
but should adopt the new API for use in new work.


Attribute Queries
=================

Please see the API Reference (mapscript.txt) for details of 
layerObj::queryByAttributes().

Spatial Queries
===============

Please see the API Reference (mapscript.txt) for details of layerObj's
queryByPoint(), queryByRect(), queryByShape(), and queryByFeatures()
methods.


