Create logic within DTD/XML

Question

Due to my studies I'm learning about XML/DTD's. Now I've got an exercise in which I have to create an order. One point is, that I have 2 different types of customers. One student and one not student. This topic, I solved with a simple boolean.

<!ATTLIST Kunde isStudent (true | false) #REQUIRED>

But now I should display additional Elements if the customer is a Student. And if he's not, they aren't required.

My current structure is like this:

<!ELEMENT Customer (Name, Address)>
<!ATTLIST Customer isStudent (true | false) #REQUIRED>

<!-- 
If "isStudent === true" the Customer-Element should also have 

<!ELEMENT StudentNumber (#PCDATA)>
<!ELEMENT UniversityName (#PCDATA)>

If "isStudent === false", they shouldn't/musn't be there
-->

<!ELEMENT Name (#PCDATA)>
<!ELEMENT Adress (Street, Number, PLZ, City)>

<!ELEMENT Street (#PCDATA)>
<!ELEMENT Number (#PCDATA)>
<!ELEMENT PLZ (#PCDATA)>
<!ELEMENT City (#PCDATA)>

So I'm wondering... is it possible to create logic within dtd's? So like "If this is true, show this..." Or is the key to create the elements with the "?" flag?

Answer 1

No, DTDs aren't powerful enough to express such constraints. There are other XML schema technologies that can do it: RelaxNG, XSD 1.1 (but not 1.0), or Schematron, for example.