XML specifications often consist of a type definition (typically, a DTD) and a set of integrity constraints. It has been shown previously that such specifications can be inconsistent, and thus it is often desirable to check consistency at compile-time. It is known that for general keys and foreign keys, and DTDs, the consistency problem is undecidable; however, it becomes NP-complete when all keys are one-attribute (unary), and tractable, if no foreign keys are used.

This paper investigates that interaction. In particular it studies constraint implication problems, which are important both in understanding the semantics of type/constraint systems and in query optimization. It shows that path constraints interact with types in a highly intricate way. For that purpose a number of results on path constraint implication are established in the presence and absence of type systems. These results demonstrate that adding a type system may in some cases simplify reasoning about path constraints and in other cases make it harder. For example, it is shown that there is a path constraint implication problem that is decidable in PTIME in the untyped context, but that becomes undecidable when a type system is added. On the other hand, there is an implication problem that is undecidable in the untyped context, but becomes not only decidable in cubic time but also finitely axiomatizable when a type system is imposed

Semi-structured data is becoming increasingly important with the introduction of XML and related languages and technologies. The recent shift from DTDs (document type definitions) to XML-Schema for XML data highlights the importance of a schema definition for semi-structured data applications.

Functional dependencies are an integral part of database theory and they form the basis for normalizing relational tables up to BCNF.

We present a technique for refining the design of relational storage for XML data based on XML key propagation. Three algorithms are presented: one checks whether a given functional dependency is propagated from XML keys via a predefined view; the others compute a minimum cover for all functional dependencies on a universal relation given XML keys. Experimental results show that these algorithms are efficient in practice. We also investigate the complexity of propagating other XML constraints to relations, and the effect of increasing the power of the transformation language. Computing XML key propagation is a first step toward establishing a connection between XML data and its relational representation at the semantic level.

XML is emerging as a new major standard for representing data on the world wide web. Several XML storage models have been proposed to store XML data in different database management systems. The unique feature of model-mapping-based approaches is that no DTD information is required for XML data storage. In this paper, we present a new modelmapping -based storage model, called XParent. Unlike the existing work on model-mapping-based approaches that emphasized on converting XML documents to/from database schema and translation of XML queries into SQL queries, in this paper, we focus ourselves on the e#ectiveness of storage models in terms of query processing. We study the key issues that affect query performance, namely, storage schema design (storing XML data across multiple tables) and path materialization (storing path information in databases). We show that similar but di#erent storage models significantly a#ect query performance. A performance study is conducted using three data sets and query sets. The experimental results are presented.

Recently, the database research literature has seen an explosion of publications with the goal of using an RDBMS to store and/or query XML data. The problems addressed and solved in this area are diverse. This diversity renders it di#cult to know how the various results presented fit together, and even makes it hard to know what open problems remain. As a first step to rectifying this situation, we present a classification of the problem space and discuss how almost 40 papers fit into this classification. As a result of this study, we find that some basic questions are still open. In particular, for the XML publishing of relational data and for "schema-based" shredding of XML documents into relations, there is no published algorithm for translating even simple path expression queries (with the // axis) into SQL when the XML schema is recursive.

The use of relational database management systems (RDBMSs) to store and query XML data has attracted considerable interest with a view to leveraging their powerful and reliable data management services. Due to the mismatch between the relational and XML data models, it is necessary to first shred and load the XML data into relational tables, and then translate XML queries over the original data into equivalent SQL queries over the mapped tables. Although there is a rich literature on XML-relational storage, none of the existing solutions addresses all the storage problems in a single framework. Works on mapping strategies often have little or no details about query translation, and proposals for query translation often target a specific mapping strategy. XML-storage solutions provided by RDBMS also have limitations. Notably, they are tied to a specific backend and use proprietary mapping languages,

A large amount of data needed in decision-making processes is stored in the XML data format, which is widely used for ecommerce and Internet-based information exchange. Thus, as more organizations view the web as an integral part of their communication and business, the importance of integrating XML data in data warehousing environments is becoming increasingly high. In this paper we show how the design of a data mart can be carried out starting directly from an XML source. Two main issues arise: on the one hand, since XML models semi-structured data, not all the information needed for design can be safely derived; on the other, different approaches for representing relationships in XML DTDs and Schemas are possible, each with different expressive power. After discussing these issues, we propose a semi-automatic approach for building the conceptual schema for a data mart starting from the XML sources.

We consider the problem of translating XML queries into SQL when XML documents have been stored in an RDBMS using a schema-based relational decomposition. Surprisingly, there is no published XML-to-SQL query translation algorithm for this scenario that handles recursive XML schemas. We present a generic algorithm to translate path expression queries into SQL in the presence of recursion in the schema and queries. This algorithm handles a general class of XML-to-Relational mappings, which includes all techniques proposed in literature. Some of the salient features of this algorithm are: (i) It translates a path expression query into a single SQL query, irrespective of how complex the XML schema is, (ii) It uses the "with" clause in SQL99 to handle recursive queries even over non-recursive schemas, (iii) It reconstructs recursive XML subtrees with a single SQL query and (iv) It shows that the support for linear recursion in SQL99 is sufficient for handling path expression queries over arbitrarily complex recursive XML schema.