Selective element based on attribute

Hi  there,

How can I define an xsd for the following scenario?

<parenttag attr="add | del | chg">
     <childtag1>chidvalue1</childtag1>
     <childtag2>chidvalue2</childtag2>
     <childtag3>chidvalue3</childtag3>
</parenttag>

Is it possible to create an xsd where childtag3 will have a value null if 
the parenttag  attr equals add else a nonnull-String when the parenttag 
attr is del/chg?

Thanks
Muthu