Hi Marcelo,
I'm not able to see your screen shot but it sounds like you can do something like this. Add a static value to your prompt (p_prompt). Set use value to ALL, and Display Value to "All". Use a query (or additional static values) to populate the other options for the prompt.
On the query you want to filter, create the filter calc like this:
if (?p_prompt? = 'ALL)
then (1=1)
else [column to filter] = ?p_Prompt?
Another option would be to do what Robert suggested, set all prompts to optional and change the display text of the prompt values to be "All". You would need to create a header or label somewhere to let users know what the prompt is for.
Hope this helps,
Todd
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Todd Schuman
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