Difference between xsd:key and xsd:unique

Question

What is the difference between using xsd:unique and xsd:key?

Answer 1

I found my own answer:

It all has to do with cardinality:

xsd:key is used for (0..N) and (1..1) multiplicity. xsd:unique is used for optional (0..1) multiplicity. This is also the reason why a the selected field in a xsd:unique can be nill.

Source of my answer: http://www.xml.com/lpt/a/987