Chapter 26 xml and Internet Databases Outline

XML Documents, DTD, and XML Schema

• A valid XML document is 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.
• Figure 26.4: a sample XML DTD called projects
• * Zero or more, + one or more, ? Zero or one
• Otherwise: exactly once
• (data type)
• (#PCDATA) parsed character data

FIGURE 26.4 An XML DTD file called projects

• To use the DTD file:
• Store the DTD file in the same file system as the XML document
• “no”?>
• “proj.dtd”>

DTD Limitations

• Data types in DTD are not very general
• Has its own special syntax and thus requires specialized processors
• All DTD elements are always forced to follow the specified ordering of the documents, so unordered elements are not permitted.
• Solution  XML Schema

FIGURE 26.5 An XML schema file called company

•  Schema namespace

•  the root element company; also an unnamed complex element

• “Department”, “Employee”, etc. must be named types.
• The selector “employeeDependent” is an attribute of “Employee”, of type “Dependent”.
• The field “dependentName” in “Dependent” must be unique.

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

• specifies a key constraint for non-primary key element.
• specifies a primary key.
• specifies a foreign key; refers to the referencing element type; refers to the referencing attribute.

FIGURE 26.5 (continued) An XML schema file called company

• Exercise: Define the element “projectWorker” in the type “Project” as an embedded sub-element.

• Answer: