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Extending ArcIMS MapServices with ArcXML
Continued...

Querying Data and Joining Tables

Querying data, based on a spatial constraint or a tabular database, is fundamental to GIS. Spatial and tabular query statements can be made using the SPATIALQUERY element. This element draws features on the map based on a query--a spatial query, a tabular query, or a combination of the two. The SPATIALQUERY also enables simple tabular joins.

Tabular query statements, located in the where attribute of the SPATIALQUERY element, define the WHERE portion of a SQL statement. The features that meet criteria set in the where attribute are drawn on the map. In query statements that require the use of symbols, such as specifying values using greater than or less than signs, the escape values for these symbols (> and < in this example) must be used inside the where attribute.

A spatial filter defines the envelope for a spatial query. The envelope can be a rectangle, point, line, polygon, or buffer. All features within the spatial filter will be drawn on the map. The SPATIALFILTER child element, used within the SPATIALQUERY element, is most commonly defined as an ENVELOPE element that uses a rectangular spatial filter or as a POLYGON element that uses a ring of points. Either will determine where the spatial filter will be drawn. The relation attribute, required by the SPATIALFILTER element, determines whether the spatial filter shape is a rectangular envelope around the extents of a shape (envelope intersection) or is limited to the shape of the spatial filter itself (area intersection).

The SPATIALQUERY element can also be used to join dBASE (DBF) files to shapefiles and to relate tables in ArcSDE based on a common field. Shapefile layers can only be joined to other DBF files located in the same directory, and ArcSDE tables must be in the same DBMS. The TO (or master) table and common field name and the FROM (or join) table and common field name and the type of join are identified inside the element. An exact join is appropriate for one-to-one or many-to-one relationships. A scan join is most effective for one-to-many or many-to-many relationships.

Conclusion

By understanding the structure of ArcXML and elements and attributes, the functionality of ArcIMS can be extended by modifying elements in the map configuration file. Try the three companion tutorials for this article. Each gives detailed instructions for manually editing the map configuration files. Read more about ArcXML and the map configuration file in the ArcXML Programmer's Reference Guide.

About the Author

Esri Educational Services team member Mark Ho is an ArcIMS and ArcView 3.x instructor and the coauthor and course manager for the three-day instructor-led course, Introduction to ArcIMS.

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