Joins

In contrast to the relational modelling for the representation of discourse areas, there is a broad range of liberties with regard to the structuring of the representation in the XML data model. Therefore, it depends on the liberty of the database design whether 1:n relations between entities are reflected structurally in a hierarchical arrangement, stand side by side within a document and joined with references or are split up in various documents.

As a consequence, the combination of data stocks from different parts of a document or of various documents is essential. A self-join was already outlined on the example of the pivoting of XML documents. The general structure of joins and the different semantics are discussed in detail on the following pages, whereby in a first step an implicit join is explained by the tracing of references.

• Tracing of references
• Symmetric join
• Left or right outer join
• Join variants
• Asymmetric join notation
• Full outer join

Source: "XQuery – Grundlagen und fortgeschrittene Methoden", dpunkt-Verlag, Heidelberg (2004)