What would be the formula so that a value entered returns all data?

Meaning, if my parameter prompts for a value to be entered, which must be a number, and a zero is entered, I need the forumula to return all data (for that parameter), based on another date parameter.

Any help is greatly appreciated.

Where would I enter this? I ask because my main report has about 9 subreports and they all have this parameter.

However, it's from the main report that you enter zero in the parameter.

Would it be possible to create another subreport that would do this?
All the subreports bring in the data in totals onto the main report. So, I would need to have the parameter bring in all data wen you enter a zero, but also I need the data grouped.

This is so confusing........help!!!!!