ENCh26.ppt

Chapter 26
XML and Internet Databases
Copyright © 2004 Pearson Education, Inc.

Copyright © 2004 Ramez Elmasri and Shamkant Navathe
Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 26-3
Chapter Outline
Introduction
Structured, Semi structured, and Unstructured
Data.
XML Hierarchical (Tree) Data Model.
XML Documents, DTD, and XML Schema.
XML Documents and Databases.
XML Querying.
– Xpath
– XQuery

Introduction
 Although HTML is widely used for formatting and structuring Web
documents, it is not suitable for specifying structured data that is
extracted from databases.
 A new language—namely XML (eXtended Markup Language) has
emerged as the standard for structuring and exchanging data over
the Web. XML can be used to provide more information about the
structure and meaning of the data in the Web pages rather than just
specifying how the Web pages are formatted for display on the
screen.
 The formatting aspects are specified separately—for example, by
using a formatting language such as XSL (eXtended Stylesheet
Language).

Structured, Semi Structured and Unstructured
Data.
 Information stored in databases is known as structured data because it is
represented in a strict format. The DBMS then checks to ensure that all data
follows the structures and constraints specified in the schema.
 In some applications, data is collected in an ad-hoc manner before it is known
how it will be stored and managed. This data may have a certain structure,
but not all the information collected will have identical structure. This type of
data is known as semi-structured data.
– In semi-structured data, the schema information is mixed in with the data
values, since each data object can have different attributes that are not
known in advance. Hence, this type of data is sometimes referred to as
self-decribing data.
 A third category is known as unstructured data, because there is very
limited indication of the type of data. A typical example would be a text
document that contains information embedded within it. Web pages in HTML
that contain some data are considered as unstructured data.

Structured, Semi Structured and
Unstructured Data (cont.)
 Semi-structured data may be displayed as a directed graph, as
shown.
 The labels or tags on the directed edges represent the schema
names—the names of attributes, object types (or entity types or
classes), and relationships.
 The internal nodes represent individual objects or composite
attributes.
 The leaf nodes represent actual data values of simple (atomic)
attributes.

FIGURE 26.1
Representing semistructured data as a graph.

XML
Hierarchical
(Tree) Data
Model
FIGURE 26.3
A complex XML
element called
<projects>.

XML Hierarchical (Tree) Data Model (cont.)
 The basic object is XML is the XML document. There are two
main structuring concepts that are used to construct an XML
document: elements and attributes. Attributes in XML provide
additional information that describe elements.
 As in HTML, elements are identified in a document by their
start tag and end tag. The tag names are enclosed between
angled brackets <…>, and end tags are further identified by a
backslash </…>. Complex elements are constructed from other
elements hierarchically, whereas simple elements contain data
values.
 It is straightforward to see the correspondence between the
XML textual representation and the tree structure. In the tree
representation, internal nodes represent complex elements,
whereas leaf nodes represent simple elements. That is why the
XML model is called a tree model or a hierarchical model.

XML Hierarchical (Tree) Data Model (cont.)
It is possible to characterize three main types of XML
documents:
1. Data-centric XML documents:
These documents have many small data items that follow a
specific structure, and hence may be extracted from a
structured database. They are formatted as XML documents in
order to exchange them or display them over the Web.
2. Document-centric XML documents:
These are documents with large amounts of text, such as news
articles or books. There is little or no structured data elements
in these documents.
3. Hybrid XML documents:
These documents may have parts that contains structured data
and other parts that are predominantly textual or unstructured.

XML Documents, DTD, and XML Schema.
Well-Formed
– It must start with an XML declaration to indicate the version of XML being used—as
well as any other relevant attributes.
– It must follow the syntactic guidelines of the tree model. This means that there should
be a single root element, and every element must include a matching pair of start tag
and end tag within the start and end tags of the parent element.
– A well-formed XML document is syntactically correct. This allows it to be
processed by generic processors that traverse the document and create an internal tree
representation.
 DOM (Document Object Model) - Allows programs to manipulate the resulting
tree representation corresponding to a well-formed XML document. The whole
document must be parsed beforehand when using dom.
 SAX - Allows processing of XML documents on the fly by notifying the
processing program whenever a start or end tag is encountered.
Valid
– A stronger criterion is for an XML document to be valid. In this case, the document
must be well-formed, and in addition the element names used in the start and end tag
pairs must follow the structure specified in a separate XML DTD (Document Type
Definition) file or XML schema file.

XML Documents, DTD, and XML Schema (cont.)
FIGURE 26.4 An XML DTD file called projects.

XML DTD Notation
 A * following the element name means that the element can be repeated zero or more
times in the document. This can be called an optional multivalued (repeating) element.
 A + following the element name means that the element can be repeated one or more
times in the document. This can be called a required multivalued (repeating) element.
 A ? following the element name means that the element can be repeated zero or one
times. This can be called an optional single-valued (non-repeating) element.
 An element appearing without any of the preceding three symbols must appear exactly
once in the document. This can be called an required single-valued (non-repeating)
element.
 The type of the element is specified via parentheses following the element. If the
parentheses include names of other elements, these would be the children of the
element in the tree structure. If the parentheses include the keyword #PCDATA or one
of the other data types available in XML DTD, the element is a leaf node. PCDATA
stands for parsed character data, which is roughly similar to a string data type.
 Parentheses can be nested when specifying elements.
 A bar symbol ( e1 | e2 ) specifies that either e1 or e2 can appear in the document.

Limitations of XML DTD
 First, the data types in DTD are not very general.
 DTD has its own special syntax and so it requires specialized
processors. It would be advantageous to specify XML schema
documents using the syntax rules of XML itself so that the same
processors for XML documents can process XML schema
descriptions.
 Third, all DTD elements are always forced to follow the specified
ordering the document so unordered elements are not permitted.

FIGURE 26.5 An XML schema file called company.

FIGURE 26.5
(continued)
An XML schema
file called
company.

FIGURE 26.5
(continued)
An XML schema file
called company.

FIGURE 26.5 (continued)
An XML schema file called company.

XML Schema
 Schema Descriptions and XML Namespaces:
It is necessary to identify the specific set of XML schema language elements
(tags) by a file stored at a Web site location. The second line in our example
specifies the file used in this example, which is:
"http://www.w3.org/2001/XMLSchema".
Each such definition is called an XML namespace.
The file name is assigned to the variable xsd using the attribute xmlns (XML
namespace), and this variable is used as a prefix to all XML schema tags.
 Annotations, documentation, amd language used:
The xsd:annotation and xsd:documentation are used for providing comments
and other descriptions in the XML document. The attribute XML:lang of the
xsd:documentation element specifies the language being used. Eg. “en”
 Elements and types:
We specify the root element of our XML schema. In XML schema, the name
attribute of the xsd:element tag specifies the element name, which is called
company for the root element in our example. The structure of the company
root element is a xsd:complexType.

XML Schema
 First-level elements in the company database:
These elements are named employee, department, and project, and each is
specified in an xsd:element tag. If a tag has only attributes and no further sub-
elements or data within it, it can be ended with the back slash symbol (/>) and
termed Empty Element.
 Specifying element type and minimum and maximum occurrences:
If we specify a type attribute in an xsd:element, this means that the structure of
the element will be described separately, typically using the xsd:complexType
element. The minOccurs and maxOccurs tags are used for specifying lower
and upper bounds on the number of occurrences of an element. The default is
exactly one occurrence.
 Specifying Keys:
For specifying primary keys, the tag xsd:key is used.
For specifying foreign keys, the tag xsd:keyref is used. When specifying a
foreign key, the attribute refer of the xsd:keyref tag specifies the referenced
primary key whereas the tags xsd:selector and xsd:field specify the referencing
element type and foreign key.

XML Schema
 Specifying the structures of complex elements via complex types:
Complex elements in our example are Department, Employee, Project, and
Dependent, which use the tag xsd:complexType. We specify each of these as
a sequence of subelements corresponding to the database attributes of each
entity type by using the xsd:sequence and xsd:element tags of XML schema.
Each element is given a name and type via the attributes name and type of
xsd:element.
We can also specify minOccurs and maxOccurs attributes if we need to change
the default of exactly one occurrence. For (optional) database attributes where
null is allowed, we need to specify minOccurs = 0, whereas for multivalued
database attributes we need to specify maxOccurs = “unbounded” on the
corresponding element.
 Composite (compound) attributes:
Composite attributes from ER Schema are also specified as complex types in
the XML schema, as illustrated by the Address, Name, Worker, and WorkesOn
complex types. These could have been directly embedded within their parent
elements.

XML Documents and Databases.
Approaches to Storing XML Documents
 Using a dbms to store the documents as text:
We can use a relational or object dbms to store whole XML documents as text
fields within the dbms records or objects. This aproach can 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 document contents as data elements:
This approach would work for storing a collection of documents that follow a
specific XML DTD or XML schema. Since all the documents have the same
structure, we can design a relational (or object) database to store the leaf-level
data elements within the XML documents.
 Designing a specialized system for storing native XML data:
A new type of database system based on the hierarchical (tree) model would
be designed and implemented. The system would include specialized indexing
and querying techniques, and would work for all types of XML documents.
 Creating or publishing customized XML documents from pre-existing
relational databases:
Because there are enormous amounts of data already stored in relational
databases, parts of these data may need to be formatted as documents for
exchanging or displaying over the Web.

Extracting XML Documents from Relational Databases.
Suppose that an application needs to extract XML documents for student,
course, and grade information from the university database. The data needed
for these documents is contained in the database attributes of the entity types
course, section, and student as shown below (part of the main ER), and the
relationships s-s and c-s between them.

FIGURE 26.7
Subset of the UNIVERSITY database schema needed for
XML document extraction.

Extracting XML Documents from Relational Databases.
One of the possible hierarchies that can be extracted from the database subset
could choose COURSE as the root.

FIGURE 26.8
Hierarchical (tree)
view with COURSE
as the root.

FIGURE 26.9
XML schema document with COURSE as the root.

Breaking Cycles To Convert Graphs into Trees
It is possible to have a more complex subset with one or more cycles, indicating
multiple relationships among the entities.
Suppose that we need the information in all the entity types and relationships in
figure below for a particular XML document, with student as the root element.

FIGURE 26.6
An ER schema diagram for a simplified
UNIVERSITY database.

Breaking Cycles To convert Graphs into Trees
One way to break the cycles is to replicate the entity types
involved in cycles.
 First, we replicate INSTRUCTOR as shown in part (2) of Figure, calling the
replica to the right INSTRUCTOR1. The INSTRUCTOR replica on the left
represents the relationship between instructors and the sections they teach,
whereas the INSTRUCTOR1 replica on the right represents the relationship
between instructors and the department each works in.
 We still have the cycle involving COURSE, so we can replicate COURSE in a
similar manner, leading to the hierarchy shown in part (3) . The COURSE1
replica to the left represents the relationship between courses and their
sections, whereas the COURSE replica to the right represents the relationship
between courses and the department that offers each course.

FIGURE 26.13
Converting a graph with cycles into a hierarchical
(tree) structure.

XML Querying
XPath
 An XPath expression returns a collection of element nodes that satisfy certain
patterns specified in the expression.
 The names in the XPath expression are node names in the XML document tree
that are either tag (element) names or attribute names, possibly with additional
qualifier conditions to further restrict the nodes that satisfy the pattern.
 There are two main separators when specifying a path:
single slash (/) and double slash (//).
A single slash before a tag specifies that the tag must appear as a direct child of
the previous (parent) tag, whereas a double slash specifies that the tag can appear
as a descendant of the previous tag at any level.
 It is customary to include the file name in any XPath query allowing us to specify
any local file name or path name that specifies the path.
doc(www.company.com/info.XML)/company => COMPANY XML doc

XML Querying
1. Returns the COMPANY root node and all its descendant nodes, which
means that it returns the whole XML document.
2. Returns all department nodes (elements) and their descendant subtrees.
3. Returns all employeeName nodes that are direct children of an employee
node, such that the employee node has another child element
employeeSalary whose value is greater than 70000.
4. This returns the same result as the previous one except that we specified the
full path name in this example.
5. This returns all projectWorker nodes and their descendant nodes that are
children under a path /company/project and that have a child node hours
with value greater than 20.0 hours.

FIGURE 26.14
Some examples of XPath expressions on XML
documents that follow the XML schema file COMPANY in
Figure 26.5.

XML Querying
XQuery
 XQuery uses XPath expressions, but has additional constructs.
 XQuery permits the specification of more general queries on one or
more XML documents.
 The typical form of a query in XQuery is known as a FLWR
expression, which stands for the four main clauses of XQuery and
has the following form:
FOR <variable bindings to individual nodes (elements)>
LET <variable bindings to collections of nodes (elements)>
WHERE <qualifier conditions>
RETURN <query result specification>

XML Querying
1. This query retrieves the first and last names of employees who earn more
than 70000. The variable $x is bound to each employeeName element that
is a child of an employee element, but only for employee elements that
satisfy the qualifier that their employeeSalary is greater that 70000.
2. This is an alternative way of retrieving the same elements retrieved by the
first query.
3. This query illustrates how a join operation can be performed by having
more than one variable. Here, the $x variable is bound to each
projectWorker element that is a child of project number 5, whereas the $y
variable is bound to each employee element. The join condition matches
ssn values in order to retrieve the employee names.

FIGURE 26.15
Some examples of XQuery queries on XML documents
that follow the XML schema file COMPANY in Figure
26.5.

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