Querying

Author:Sean Gillies

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.

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)

        layer.getFeature(result)

        ...  # do something with this feature

    # Close when done
    layer.close()

Backwards Compatibility

The API changed substantially with version 6.0 and backward compatibility was broken. Scripts will have to be updated to work with the new API.

Attribute Queries

By Attributes

queryByAttributes()

Spatial Queries

By Rectangle

queryByRect()

By Point

queryByRect()

By Shape

queryByShape()

By Selection

queryByFeatures()