[xml-dev] Key constraint on optional attribute

[Paul Popiel <PaulP@advancedengine.com>]

Hi,

is there anyway of placing a key constraint on an optional attribute?

namely, if the optional attribute is defined for the element then it must be
unique compared to the other, optionally, defined attributes (same
attribute).

ive tried xpath expressions such as ".//elementName[@OptAttr]"  for the
selector, but i get an error along the lines of "invalid XPATH for selector
or field"

thanks in advance

paul


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