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XML elements can have attributes in the start
tag, just like HTML.
Attributes are used to provide additional
information about elements.
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">
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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>
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Quote Styles, "female" or 'female'?
Attribute values must always be enclosed in quotes, but
either single or double quotes can be used. For a person's sex,
the person tag can be written like this:
or like this:
Note: If the attribute value itself contains double
quotes it is necessary to use single quotes, like in this
example:
<gangster name='George "Shotgun" Ziegler'>
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Note: If the attribute value itself contains single
quotes it is necessary to use double quotes, like in this
example:
<gangster name="George 'Shotgun' Ziegler">
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Use of Elements vs. Attributes
Data can be stored in child elements or in attributes.
Take a look at these examples:
<person sex="female">
<firstname>Anna</firstname>
<lastname>Smith</lastname>
</person>
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<person>
<sex>female</sex>
<firstname>Anna</firstname>
<lastname>Smith</lastname>
</person>
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In the first example sex is an attribute. In the last, sex is
a child element. Both examples provide the same information.
There are no rules about when to use attributes, and when to
use child elements. My experience is that attributes are handy
in HTML, but in XML you should try to avoid them. Use child
elements if the information feels like data.
My Favorite Way
I like to store data in child elements.
The following three XML documents contain exactly the same
information:
A date attribute is used in the first example:
<note date="12/11/2002">
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
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A date element is used in the second example:
<note>
<date>12/11/2002</date>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
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An expanded date element is used in the third: (THIS IS MY
FAVORITE):
<note>
<date>
<day>12</day>
<month>11</month>
<year>2002</year>
</date>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
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Avoid using attributes?
Should you avoid using attributes?
Some of the problems with using attributes are:
- attributes cannot contain multiple values (child
elements can)
- attributes are not easily expandable (for future
changes)
- attributes cannot describe structures (child elements
can)
- attributes are more difficult to manipulate by program
code
- attribute values are not easy to test against a Document
Type Definition (DTD) - which is used to define the legal
elements of an XML document
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 (this is not how XML should be used):
<note day="12" month="11" year="2002"
to="Tove" from="Jani" heading="Reminder"
body="Don't forget me this weekend!">
</note>
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An Exception to my Attribute Rule
Rules always have exceptions.
My rule about attributes has one exception:
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="p501">
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
<note id="p502">
<to>Jani</to>
<from>Tove</from>
<heading>Re: Reminder</heading>
<body>I will not!</body>
</note>
</messages>
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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.
What I am trying to say here is that metadata (data about
data) should be stored as attributes, and that data itself
should be stored as elements. |
