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<p><b>Abstract</b>—An important trend of current GIS development is to provide easy and effective access to spatial analysis functionalities for supporting decision making based on geo-referenced data. Within the framework of the ongoing SQL standards for spatial extensions, a spatial query language, called SQL/SDA, has been designed to meet such a requirement. Since the language needs to incorporate the important derivation functions (e.g., map-overlay and feature-fusion) as well as the spatial relationship and metric functions, the functionality of the FROM clause in SQL is developed in addition to the SELECT and WHERE clauses. By restructuring the FROM clause via a subquery, SQL/SDA is well-adapted to the general spatial analysis procedures using current GIS packages. Such an extended SQL, therefore, stretches the capabilities of the previous ones. The implementation of SQL/SDA on the Internet adopts a hybrid model, which takes advantage of the Web GIS design methods in both the client side and server side. The client side of SQL/SDA, programmed in the Java language, provides a query interface by introducing visual constructs such as icons, listboxes, and comboboxes to assist in the composition of queries, thereby enhancing the usability of the language. The server side of SQL/SDA, which is composed of a query processor and Spatial Database Engine (SDE), carries out query processing on spatial databases after receiving user requests. It was demonstrated that using the familiar SELECT-FROM-WHERE statement instead of a single ad hoc command or procedural commands like macro language in some GIS packages, SQL/SDA offers users an efficient option to perform complicated multistep spatial data analyses on the Internet.</p>
Geographical Information System (GIS), spatial analysis, query languages, spatial database, SQL, Internet, client/server, visual interface, Java.

H. Lin and B. Huang, "SQL/SDA: A Query Language for Supporting Spatial Data Analysis and Its Web-Based Implementation," in IEEE Transactions on Knowledge & Data Engineering, vol. 13, no. , pp. 671-682, 2001.
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