this part is about how xml documents can be stored and retrieved

XML DOCUMENTS AND DATABASES

This part is about how XML Documents can be stored and retrieved.

Introduction

Several approaches to organizing the contents of XML documents to facilitate their subsequent querying and retrieval have been proposed. The following are the most common approaches.

Approaches to Storing XML Documents

A relation or object DBMS can be used to store whole XML documents as text fields within the DBMS records or objects.

This approaches cen be used if the DBMS has a special module for document processing, and would work for storing schemaless and document- centric XML documents.

Using a DBMS to store the documents as text

The keyword indexing functions of the document processing module can be used to index and speed up search and retrieval of the documents

Using a DBMS to store the documents as text

This approach would work for storing a collection of documents that follow a specific XML DTD or XML schema.

Because all the documents have the same structure, one can design a relational database to store the leaf data elements within the XML documents.

Using a DBMS to store the document contents as data elements

To be able to handle this it requires mapping algorithmsto design a database schema that is compatible with the XML document structure as specified in the XML schema to recreate the XML documents from the stored data.

Using a DBMS to store the document contents as data elements

A new type of database system based on the tree model could be desinged and implemented.

The system would include specialized indexing and would work for all types of XML documents.

By the way you can use compression techniques in this approach.

Designing a speciliazed system for storing native XML data

There are enormous amounts of data already stored in relational databases.

Parts of this data may need to be formatted as documents for exchanging or displaying over the Web

Creating and publishing customized XML documents from preexisting relational databases

This approach would use a seperate middleware software layer to handle the conversions needed between the XML documents and teh relational database.

Creating and publishing customized XML documents from preexisting relational databases

Extracting XML Documents from Relational Databases

In this part we focused on the last approach.

XML uses tree model to represent documents. When we add referential integrity constraints, a relational schema can be considered to be a graph structure.

Similarly also the ER model represents data using graphlike structures

There would be straightforward mappings between the ER and relational models.


So we can conceptually represent a relational database schema using the corresping ER schema.

Namely the issue is clarifying differences between tree and graph models. If we solve this also solve the problem of converting relational data to XML.


Objective: Creating application to extract XML documents for student course and grade information from University DB.

Example

Example

To be able to do this we need COURSE, SECTION and STUDENT attributes.

Example

Example

When we convert it to tree model we must select root. So in this example we can choose three different root.

Example

Example

It is possible to have more complex subset with one or more cycles, indicating multiple relationships among entities

In this case it is more complex to decide how to create the document hierarchies.

Additional duplication of entities may be needed to represent the multiple relationships.

Breaking Cycles to Convert Graphs into Trees

The way is to replicate the entity types involved in the cycles.

Breaking Cycles to Convert Graphs into Trees

Example

It is necessary to create the correct query in SQL to extract the desired information for the XML document

Once the query is executed, its result must be structured from the flat relational form to the XML tree structure.

Other Steps for Extracting XML Documents from Databases