HTML vs. XML In the case of HTML, browsers have been taught how to ignore invalid HTML such as the <mymadeuptag> element and generally do their best

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3 HTML vs. XML In the case of HTML, browsers have been taught how to ignore invalid HTML such as the <mymadeuptag> element and generally do their best when dealing with badly placed HTML elements. The XML processor, on the other hand, can not tell us which elements and attributes are valid. As a result we need to define the XML markup we are using. To do this, we need to define the markup language s grammar. 3

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6 In DTD notation, we define an element class that contains 1 to many students. Students can only be of type character data (PCDATA). PCDATA is the only choice available in DTD notation <!ELEMENT class(student+)> <!ELEMENT student(#pcdata)> In DTD s the following cardinality is only supported? -> 0:1 * -> 0:m + -> 1:m IN XMLSchema we can define a element student to be of datatype string and thee class can only accommodate 30 students in our example. Note there are many types available to us e.g. integer and we will see these later. Assuming we have defined the class element, the student element could be defined as <xsd:element name= student type= xsd:string maxoccurs= 30 /> 6

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8 There are a few important things to note: 1. When creating a XML Schema document, you must begin the document with the XML processing instruction and specify the version of XML that the document is based on: <?xml version= 1.0?> 2. After including the XML PI (processing instruction), you must then initiate the XML schema document using the <schema> element. The remainder of the document will be encased inside this element. <?xml version= 1.0?> <schema xmlns:xsd=" <! your statements go here --> </schema> 3. Including the <schema> element as we have done is fine but in the case of adding additional namespaces to the document, we need to expand the notation and specify the use of the XML Schema namespace so there is no confusion within the document as we see in the above example. In addition to specifying that we are creating an element, we must also specify the type of content it will contain. In the case of a root element, the content type will be COMPLEX and we must nest the <complextype> element within the <element> element. Next we must define the contents of the <complextype> element. Since it would be pointless to have a root-element without children, we must now nest a <sequence> element within the <complextype>. And FINALLY, we can specify the contents that will occur within the root element using another <element> element. 8

9 Defining children elements in XML Schema is similar to defining a root element with a few twists. At this point, we will assume that the only datatype available is the string datatype which we will use in the following examples. An elements content may be defined or specified in several ways. The first way is to specify it within the <element> element itself using the type attribute. 9

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12 To define an element so that it can be optionally included, you must include attribute minoccurs within the <element> element and specify a value of 0. 12

13 To define an element so that it can be optionally included, you must include attribute maxoccurs within the <element> element and specify a value of unbounded. 13

14 To define an element so that it can be included zero or more times, both the minoccurs and maxoccurs attributes must be added along with the values 0 and unbounded respectively. 14

15 NOTE: minoccurs must be less or equal than maxoccurs. <xsd:element name= club-database > <xsd:complextype> <xsd:sequence> <xsd:element name= club type= xsd:string minoccurs= 2 maxoccurs= 3 /> </xsd:sequence> </xsd:complextype> </xsd:element> 15

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18 The minoccurs and maxoccurs attributes may be added to the <choice> element to specify a repetition of choices. 18

19 A group of elements can be specified within a sequence using an additional <sequence> element which must appear together. 19

20 The minoccurs and maxoccurs attributes may be added to the <sequence> element to specify a certain number of repetitions. 20

21 You can create your own data types. In effect this is a USER DEFINED TYPE Or ABSTRACT DATA TYPE you have seen in other paradigms. 21

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23 Attributes may also be defined using a user-defined type which we will see defined later. <xsd:element name= catalog > <xsd:complextype> <xsd:attribute name= catid type= userdeftype /> </xsd:complextype> </xsd:element> <catalog></catalog> <catalog catid= 123 ></catalog> Attributes may be defined as being optional or required within an <element> which is done by including the use attribute within the <attribute> element. By default the value of use is optional but if we wish to make the attribute required, we must change it to required <xsd:element name= catalog > <xsd:complextype> <xsd:attribute name= catid type= xsd:string /> </xsd:complextype> </xsd:element> 24

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26 In addition to using defined types in the type attribute of the <attribute> or <element> elements, we can also create more complex pattern, etc using a <simpletype>. The <simpletype> element must be nested within either the <element> or <attribute> element. 27

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28 Example 1 restricts marks element to the integer range of 0 to 100. Example 2 creates our own UDT to a list of string values 29

29 If you have an instance where you have an element leaf node that requires customizing which also has an attribute, you will also need a <complextype> element.

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31 We will begin creating a reference by first extracting the nested element and pasting it elsewhere in the schema document. 32

32 In general, the value of the ref attribute must reference a global element, i.e. one that has been declared under schema rather than as part of a complex type definition. 33

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34 We will begin creating user-defined type by first extracting the nested <complextype> element and pasting it elsewhere in the XML Schema document. We will now name the <complextype> element so that the now empty <element> element can refer to it later. And finally.. we need to add a type attribute to the <element> element with the name of the new user-defined type and make the element empty. 36

35 We will now name the <complextype> element so that the now empty <element> element can refer to it later. And finally.. we need to add a type attribute to the <element> element with the name of the new user-defined type and make the element empty. 37

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