SQL Server 2008 : Explaining XML - Well-Formed XML

2/15/2011 7:49:31 PM
XML is a framework for storing information in a tree. The XML document must exactly have one root tag and must have a start-tag (< >) and end-tag (</>). An XML document must adhere to the XML syntax rules, or it is not a valid document. Figure 1 is a well-formed XML document.
Figure 1. A Well-Formed XML Document
<name>Steve Long</name>

In addition, text can be contained in the root tag. These are called attributes. For example, Figure 2 shows a sample XML document in which attributes are contained in the root tag.

Figure 2. Using Attributes in an XML Document
<car make="chevrolet">

XML requires that you properly nest elements in your XML document. Overlapping cannot occur, and as mentioned earlier, you need to end your XML with the end-tag (</>). Figure 3 is not a well-formed XML document. You will see that the <year> element overlaps the <car> element.

Figure 3. Not a Well-Formed XML Document

XML also has the flexibility to contain empty data. For example, in Figure 4, you will see that there is no price available for the car. An empty-element tag resembles a start-tag but contains a slash just before the closing angle bracket.

Figure 4. Empty Data Usage
<car make="chevrolet">
<price />

Notice that the price does not have an end-tag (</>). That’s because in XML there are different ways to specify the ending of a tag.

<price />

XML documents must conform to syntax rules. Here are some of the rules to follow when you are creating a well-formed XML document.

  • All attribute values are quoted with either single (‘) or double (“) quotes. Single quotes close a single quote, and double quotes close a double quote.

  • Tags may be nested but must not overlap. Each nonroot element must be completely contained in another element.

  • Empty elements may be marked with an empty-element (self-closing) tag, such as <NoData/>. This is equal to <NoData></NoData>.

  • Element names are case-sensitive.

XML Examples

<name>Steve Long</name>

<car make="chevorlet">

<customer id="17" firstname="Bob" lastname="Smith">
<address type="home" address1="763 Main Street" city="Anytown"
state="CA" zipcode="93762"/>
<order id="17">
<line_item part_no="12" qty="1" price="12.99"/>
<line_item part_no="73" qty="2" price="6.95"/>
<line_item part_no="17" qty="1" price="2.95"/>
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