XML Schema - child elements dependant on existance of optional attribute

Question

Is it possible to define in XSD the following scenario:

1. Parent element has an attribute that is optional.
2. If the attribute is not present in XML, at least one child element must exists.
3. If the attribute is present, there can be zero or more child elements.

Example:

VALID

<parent external-source="some_name" />

<parent external-source="some_name">
  <child>some value</child>
</parent>

<parent>
  <child> some value</child>
</parent>

NOT VALID

<parent />

Answer 1

No i think not.

Answer 2

No .. the reason is : In your case you are trying to validate the presence of an element/tag depending on the value of some other tag/attribute .. (XSD is basically a set of declaration) which requires multiple declaration of a same element ..
Multiple declaration of a same element isn't allowed in XSD .. :-(
Check out the similar problem (click here) posted by a stackOverFlow member