Difference between xsd:key and xsd:unique

Question

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

Answer 1

xsd:unique does not require the field to exist for the selector. If the field exists, its value should be unique.

xsd:key requires the field to exist for all selector results. Every element in the selector should have a key.

Note that xsd:keyref is not a requirement for xsd:key, the xsd:key can be there withoug any xsd:keyref.