I don't believe this is directly possible in SMW. I can think of two workarounds, though:

- You can create a new Boolean semantic property, perhaps called "Has effort value", that is set using an #if call; and then query on that property. This is easier if you're using templates to store the data, but if not, you can use a call to #show to get the value.

- The Semantic Drilldown extension can display the pages that have no value for a property.

-Yaron


On Wed, Feb 25, 2009 at 10:59 AM, planetenxin <planetenxin@web.de> wrote:

Hi forum,

i have properties that are not mandatory and therefore sometimes populated
and sometimes not. I'd like to retrieve all pages with an "empty" property
e.g. all pages where the optional property "Effort" is not set:

{{#ask: [[Category:Requirement]][[Effort::NULL]]
| ? Realized By Project
| ? Binding Character
| ? Process Word
| ? Effort = Effort (MD)
}}

There are also other examples where the datatype is a string or text, e.g.
"Realized By Project"

/planetenxin
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