Databases: A Beginner s Guide in Software

Creator UPC-A Supplement 2 in Software Databases: A Beginner s Guide

As mentioned, XML is commonly used to represent data on web pages, and that data often comes from relational databases However, as you have seen, the two models in use are quite different, in that relational data is stored in tables where neither hierarchy nor sequence have any significance, while XML is based on hierarchical trees in which order is considered significant The term forest is often used to refer to a collection of XML tree structures XML is used for web pages because its structure so closely matches the structure that would be used to display the same data in HTML In fact, many web pages are a mixture of HTML for the static portions and XML for the dynamic data It is perhaps this widespread implementation that has led many of the major vendors, including Oracle, Microsoft, and IBM, to support XML extensions However, only Oracle and IBM s DB2 UDB support the SQL/XML commands covered in this topic the Microsoft SQL Server XML extension is markedly different, and I have not included it in this book because it is proprietary SQL/XML can be divided into three main parts: the XML data type, SQL/XML functions, and SQL/XML mapping rules I cover each of these as the major topics in the remainder of this chapter

The XML data type is handled in the same general way as all the other data types discussed in 2 Storing data in XML format directly in the database is not the only way to use SQL and XML together However, it is a very simple way to get started, because it is a logical extension of the earliest implementations where SQL developers simply stored the XML text in a column defined with a general character data type such as CHARACTER VARYING (VARCHAR) It is far better to tell the DBMS that the column contains XML, and the particular way the XML is coded, so that the DBMS can provide additional features tailored to the XML format The specification for the XML data type has this general format: XML ( <type modifier> {( <secondary type modifier> )} ) The type modifier is required and must be enclosed in a pair of parentheses as shown, while the secondary type modifier is optional, and in fact is not supported for all type modifiers The standard is not specific about how a particular SQL implementation should treat the various types, but some conventions and syntax rules are specified The valid type modifiers are as follows:

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DOCUMENT The DOCUMENT type is intended for storage of text documents formatted using XML In general, the data values are expected to be composed of human-readable characters such as letters, numbers, and symbols as they would appear in an unstructured text document CONTENT The CONTENT type is intended for more complex data that can include binary data such as images and sound clips SEQUENCE The SEQUENCE type is intended for XQuery documents, which are often called XQuery sequences XQuery is an advanced topic that is beyond the scope of this book

The secondary type modifier, used only with the DOCUMENT and CONTENT primary type modifiers, can have one of these values:

XMLSCHEMA The XMLSCHEMA type refers to a registered XML schema that has been made known to the database server The three most common are shown in the following table: Common Prefix

Target Namespace URI (Uniform Resource Identifier)

For SQL implementations that do not support the secondary type modifier, ANY is assumed as a default

Because SQL/XML is a relatively new standard, vendor implementation support varies Oracle supports a XMLType data type instead of the XML type, but it applies at the table level so that the entire table is stored as XML IBM s DB2 UDB supports an XML type, but without the type modifiers As mentioned, Microsoft SQL Server supports XML and an XML data type, but in a manner a bit different from the SQL/XML standard As of version 50, MySQL provides no support for XML, but it is expected to be included in a future release

Suppose we want to add the course syllabus to our course table that can be displayed on a web page If the syllabus could come from several different sources, and thus be