What is Joins Operator

An interesting feature of GML is to consider it as a database, but only in the strictest sense of the term. That is, as a collection of data. As a database format, it can be queried. In order to do this, we need a query language with spatial operators. In addition, in order to use any query language over GML, it is necessary to find an implementation that allows to exploit all its features, i.e., an efficient storage of GML documents is necessary. The general aim of this chapter is to discuss different approaches for storing and querying GML documents. In order to achieve our aim we discuss well-known approaches to the storage of XML documents (with only alphanumeric data) and their application to GML documents. Although there are many approaches to storing and retrieving XML documents with only alphanumeric features, few approaches are applicable to query GML documents.