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Top1. Introduction
Like managing traditional database, the management of XML documents require capabilities to handle with integrity, consistency, data dependency, data redundancy and anomalies (Arenas & Libkin, 2004; Dobbie et al., 2000; Embley & Mok, 2001; Vincent et al., 2004, 2007; Wang & Topor, 2005; Yu & Jagadish, 2008). Amongst the important problem related to XML design that needed to be looked into are data redundancy and update anomalies (Arenas & Libkin, 2004; Yu & Jagadish, 2008). Data redundancy and anomalies can happen in XML document if the schemas: DTD (Document Type Definitions) (Powell, 2007) or XML schema (Tompson et al., 2008), are not well-designed. These schemas allow one to define the structure, the content and the semantic of XML document. In this work, we consider only DTD since it is expressive enough for a large variety of applications and has been widely studied in database theory (Arenas & Libkin, 2004).
The design problem for XML document has recently been a subject of interest for database researchers, and a number of normal forms have been proposed to reduce data redundancies caused by functional or multi-valued dependencies (Arenas & Libkin, 2004; Feng et al., 2002; Mani et al., 2001;Vincent et al., 2004; Wang & Topor, 2005; Yu & Jagadish, 2008). However, XML normal form XNF, proposed by (Arenas & Libkin, 2004) achieves the best possible design form the point of view of eliminating redundancies in XML documents (Kolahi, 2007). But, there are problems with this definition of normal forms and normalization process. Firstly, the notions of XML normal forms are presented in a difficult term to be understood and hard to be implemented practically because of the lack of graphical notations for the proposed theories. Secondly; the current approach of XML normal form did not show the tremendous benefit to practitioners and thirdly; the normalization algorithms only works for the existing normal form which has a very limited semantic expressiveness.
In this paper, we propose to improve and simplify XML structural design and normalization process using graphical model called GN-DTD (Zainol & Wang, 2010). We define multi levels normal forms for GN-DTD to allow users to find an 'optimal' structure of XML elements/attributes and to produce a correct, complete and consistent representation of the real world XML data which may interest to the user.
GN-DTD is a graphical modelling approach for describing XML documents. For GN-DTD itself, we define a complete set of syntax and structure which incorporate of attribute identity, simple data type, complex data type and relationship types among elements. Furthermore, semantic constraints are also precisely defined in order to capture semantic meaning among those defined objects. The significance of using GN-DTD model is that, it helps user to arrange the content of XML document in order to give a better understanding of its corresponding DTD structures. DTD commonly represented as textual representation, hence it is particularly difficult to be analysed and understood. In practise, it often causes difficulties when even designing a simple DTD. This is partly due to textual form of the grammar itself. Because of using these GN-DTD's notations, we found it is particularly easily to improve XML structural design and more importantly, make the XML normalization procedure simpler and practical.