Database Reference
In-Depth Information
statements in Figure 9-9, there are three groups of state-
ments, one group for each record from the Rep table. Each statement group starts with a
In between the
dataroot
and
/dataroot
<
>
<
>
Rep
tag and
<
>
ends with a matching closing
/Rep
tag; those tags identify the beginning and end of one Rep record. User-
<
>
defined tag pairs (such as
) enclose field values, which are called element values (such
as 0.05, 0.07, and 0.05) from the Rep records. Each tag must have a matching closing tag in an XML
document.
Web pages continue to be written in HTML, but the last W3C recommendation was for HTML 4.01 in
1999. Since then, W3C has focused on recommendations for Extensible Hypertext Markup Language
(XHTML), which is a markup language based on XML and, thus, is a stricter version of HTML. Web browsers
continue to support HTML and have been slow to adapt to the XHTML specification. However, as more
organizations use XML, more XHTML-based Web pages will be created and used on the Internet.
An XML document contains element tags and element values. How does an XML processor understand
the meaning of the tags and the characteristics and structure of the data in an XML document? You use
either a Document Type Definition or an XML schema to provide those important facts about the data. A
Document Type Definition (DTD) specifies the elements (tags), the attributes (characteristics associated with
each tag), and the element relationships for an XML document. The DTD can be a separate file with a .dtd
extension, or you can include it at the beginning of an XML document. An XML schema is a newer form of
DTD that more closely matches database features and terminology; you can embed it at the beginning of an
XML document or place it in a separate file with an .xsd extension. Figure 9-10 shows the portion of an XML
schema specifying the characteristics of the Rate field from the Rep table. Notice how closely the attributes
for the Rate element in the XML schema match the properties for the Rate field in the Rep table.
Rate
and
/Rate
<
>
<
>
294
<xsd:element name="Rate" minOccurs="0" jetType="double"
sqlSType="float" type="xsd:double">
<xsd:annotation>
<xsd:appinfo>
<fieldProperty name="ColumnWidth" type="3" value="840"/>
<fieldProperty name="ColumnOrder" type="3" value="0"/>
<fieldProperty name="ColumnHidden" type="1" value="0"/>
<fieldProperty name="DecimalPlaces" type="2" value="255"/>
<fieldProperty name="Required" type="1" value="0"/>
<fieldProperty name="DisplayControl" type="3" value="109"/>
<fieldProperty name="TextAlign" type="2" value="0"/>
<fieldProperty name="AggregateType" type="4" value="-1"/>
</xsd:appinfo>
</xsd:annotation>
</xsd:element>
FIGURE 9-10
XML schema for the Rate element from the Rep table
XML documents contain data; DTDs and XML schemas define the structure, characteristics, and relation-
ships of the data in an XML document. Also, XHTML documents focus on data, not on presentation details. The
presentation aspects of an XML or XHTML document can be described by a stylesheet. The Extensible Style-
sheet Language (XSL) is a standard W3C language for creating stylesheets for XML documents; a stylesheet is a
document that specifies how to process the data contained in another document and present the data in a Web
browser, in a printed report, on a mobile device, in a sound device, or in other presentation media. A related
W3C standard language is XSL Transformations (XSLT), which defines the rules to process an XML document
and change it into another document; this other document may be another XML document, an XSL document,
an HTML or XHTML document, or most any other type of document.
