2.5 Exercises 49
For example, suppose that it is possible for thedateCreatedattribute to be
not applicable to a citation. Simply omitting this attribute would mean that
it has a value which is unknown rather than that the attribute does not apply
to the citation. To allow for such special cases, one can add additional values
to a datatype by using aunionas follows:
<element name=’DateCreated’>
<simpleType>
<xsd:union memberTypes=’xsd:date’>
<xsd:simpleType>
<enumeration value=’N/A’/>
</xsd:simpleType>
</xsd:union>
</simpleType>
</element>
Summary
- XSD provides built-in datatypes for the most commonly used purposes,
such as strings, numbers, dates, times, and resource references (URIs).
- New datatypes can be defined by restricting another datatype, combining
several datatypes (union), or allowing a sequence of values (list).
2.5 Exercises
- Convert the molecule DTD shown in figure 1.6 to an XML schema.
- Revise the molecule schema in exercise 2.1 above so that theelementType
attribute can only be one of the standard abbreviations of the 118 currently
known elements in the periodic table.
- Define a simple datatype for a single DNA base. Hint: Use an enumera-
tion as in exercise 2.2 above.
- Define a simple datatype for a DNA sequence.
- Define a more realistic datatype for a DNA sequence. It is a common
practice to break up amino acid and DNA sequences into more manage-
able pieces. For example, the following is a sequence in the European
Molecular Biology Laboratory (EMBL) database:
