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Create Quick Response Code in Software PART VI

PART VI
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Part VI:
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Document Type Definitions (DTDs)
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The earliest attempt to standardize XML metadata was contained in the Document Type Definition (DTD) capability of the original XML 1.0 specification. DTDs are used to specify the form and structure of a particular type of document (such as a purchase order document or a transfer-of-funds document). Figure 25-6 shows a DTD that might be used for a simple purchase order document in Figure 25-5. This DTD demonstrates only a fraction of the full capabilities of DTDs, but it illustrates the key components of a typical DTD. The !ELEMENT entries in the DTD define the element hierarchy that gives the document its basic form. DTDs provide for these different types of elements: Text-only element The element contains only a text string, which can represent a data value from a single column of database data. Element-only element The element s contents are other elements (subelements); it is the parent in a local parent/child hierarchy of elements. This type of element can be used to represent a row of a table, with subelements representing the columns. Mixed-content element The element can contain a mixture of interspersed text contents and subelements. This type is not typically used for database contents, because this mix of subelements and data doesn t naturally appear in the row/ column structure of tables. Empty-content element The element has no content neither subelements nor text content but it may have attributes. This type of element can represent a row of a table when its attributes are used to represent individual column values. Any-content element The element has unrestricted content. The content may be empty or may contain a mix of subelements and/or text. Like the mixed-content element, this type is typically not useful for XML documents used in database processing.
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<!ELEMENT purchaseOrder (customerNumber, orderNumber, orderDate, terms, orderItem*)> <!ELEMENT customerNumber (#PCDATA)> <!ELEMENT orderDate (#PCDATA)> <!ELEMENT repNumber (#PCDATA)> <!ELEMENT terms EMPTY> <!ATTLIST terms ship CDATA bill CDATA#REQUIRED> <!ELEMENT orderItem (mfr, product, qty, amount)> <!ELEMENT mfr (#PCDATA)> <!ELEMENT product (#PCDATA)> <!ELEMENT qty (#PCDATA)> <!ELEMENT amount (#PCDATA)>
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FIGURE 25-6
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DTD for a simple purchase order document
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25:
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SQL and XML
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In the purchase order DTD of Figure 25-6, the top-level purchaseOrder element and the orderItem element have the element-only type. Their declarations list the subelements that they contain. The customerNumber and orderDate elements are text-only elements, indicated by the #PCDATA definitions. The terms element is empty; it only has attributes. Both attributes have values that are character data (indicated by the CDATA type); one is required, as indicated, and the other is optional. Note that this DTD combines a data-aselements style (for the customer information) and a data-as-attributes style (for the order terms) only for illustrative purposes. In practice, you would choose one or the other style of data representation and use it consistently, to simplify processing. Document Type Definitions are critical to make XML actually useful in practice for representing structured documents for data exchange. They allow you to define the essential elements of a transactional document, such as a purchase order or an employee personnel action form or a request-for-quote form. With a DTD for such a document in place, it is straightforward to validate that a document that originates somewhere else within a company, or even outside a company, is a valid document of the specific type and can be processed. Any XML parser, whether based on the DOM API or the SAX API, is capable of validating an XML document against a supplied DTD. In addition, it s possible to explicitly declare the DTD to which an XML document should conform within the document itself. Document Type Definitions have some drawbacks, however. They lack the strong data typing typically found in relational databases. There is no way to specify that an element must contain an integer or a date, for example. DTDs also lack good support for userdefined (or corporate-defined) types or subdocument structures. For example, it s possible that the orderItem element in Figure 25-6 will appear not only in a purchase order document, but also in a change order document, an order cancellation document, a backorder document, and an order acknowledgement document. It would be convenient to define the orderItem substructure once, give it a name, and then refer to it in these other document definitions, but DTDs don t provide this capability. DTDs are also somewhat restrictive in the types of content structures that they allow, although in practice, they are usually rich enough to support the kinds of transactional documents needed for hybrid database/XML applications. Finally, the expressions used by DTDs to define document structure are an extended form of Backus Naur Form (BNF). (An example of this is the asterisk that appears after the orderItem declaration within the purchaseOrder element list in Figure 25-6, which means, This element may be repeated zero or more times. ) While familiar to computer science students who deal with computer languages, this format is unfamiliar to people who approach XML from the document markup world of HTML. All of these deficiencies appeared soon after the adoption of XML 1.0, and work to define a stronger metadata capability for XML documents began. Eventually, these efforts resulted in the XML Schema specification, described in the next section.
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