[XML Attributes][Well Formed XML][DTD/CDATA]
XML Attributes
XML elements can have attributes.
From HTML you will remember this: <IMG SRC="computer.gif">. The SRC attribute provides additional information about the IMG element.
In HTML (and in XML) attributes provide additional information about elements:
<img src="computer.gif">
<a href="demo.asp">
Attributes often provide information that is not a part of the data. In the example below, the file type is irrelevant to the data, but important to the software that wants to manipulate the element:
<file type="gif">computer.gif</file>
Use of Elements vs. Attributes
Data can be stored in elements or in attributes.
Take a look at these examples:
<person sex="female">
<firstname>Anna</firstname>
<lastname>Smith</lastname>
</person>
<person>
<sex>female</sex>
<firstname>Anna</firstname>
<lastname>Smith</lastname>
</person>
In the first example sex is an attribute. In the last, sex is an element. Both examples provides the same information.
There are no rules about when to use attributes, and when to use elements. My experience is however; that attributes are handy in HTML, but in XML you should try to avoid them. Use elements if the information feels like data.
My Favorite Way
I like to store data in elements.
The following three XML documents contain exactly the same information:
A date attribute is used in the first example:
<note date="12/11/99">
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
A date element is used in the second example:
<note>
<date>12/11/99</date>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
An expanded date element is used in the third: (THIS IS MY FAVORITE):
<note>
<date>
<day>12</day>
<month>11</month>
<year>99</year>
</date>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
Avoid using attributes?
Should you avoid using attributes?
Here are some of the problems using attributes:
attributes can not contain multiple values (elements can)
attributes are not easily expandable (for future changes)
attributes can not describe structures (child elements can)
attributes are more difficult to manipulate by program code
attribute values are not easy to test against a DTD
If you use attributes as containers for data, you end up with documents that are difficult to read and maintain. Try to use elements to describe data. Use attributes only to provide information that is not relevant to the data
Don't end up like this ( if you think this looks like XML, you have not
understood the point):
<note day="12" month="11" year="99"
to="Tove" from="Jani" heading="Reminder"
body="Don't forget me this weekend!">
</note>
An Exception to my Attribute rule
Rules always have exceptions.
My rule about attributes has one too:
Sometimes I assign ID references to elements. These ID references can be used to access XML elements in much the same way as the NAME or ID attributes in HTML. This example demonstrates this:
<messages>
<note ID="501">
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
<note ID="502">
<to>Jani</to>
<from>Tove</from>
<heading>Re: Reminder</heading>
<body>I will not!</body>
</note>
</messages>
The ID in these examples is just a counter, or a unique identifier, to identify the different notes in the XML file, and not a part of the note data.
