Parameter driven conditional SQL query

I have an ASP.NET front end app that, at one point, generates Crystal Reports for client orders.
This reporting has just been extended to give the user the ability to filter the results.

Previously this data was filtered by date only. In crystal this query was something like:

select * from orders
where ordr_conf_d between '2010-09-01' and '2010-09-30'

The user will now need to see client orders only for a selected region.
By adding a region dropdown to this, the query will need to be updated to something like this:

select * from orders
where ordr_conf_d between '2010-09-01' and '2010-09-30'
and regn_i = 5

If no region is selected the region id will default to 0 and I would like to run the first query.
How do I achieve this? The following is not possible.

select * from orders
where ordr_conf_d between '2010-09-01' and '2010-09-30'
and regn_i IN (case when @RegionID > 0
                     then @RegionID
               else (select regn_i from lkup_regn)
               end)
dbasplusAsked:
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aelliso3Connect With a Mentor Commented:
In the code below, if @RegionID  = 0 then it will just skip the regn_i because of the OR statement in there
select * from orders
where ordr_conf_d between '2010-09-01' and '2010-09-30'
      and (regn_i = 5
           or @RegionID = 0)

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cyberkiwiCommented:
select * from orders
where ordr_conf_d between '2010-09-01' and '2010-09-30'
and (@RegionID = 0 or regn_id = @RegionID)
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dbasplusAuthor Commented:
Brilliant and simple, just what I needed.

This is the final result:

select * from orders
where ordr_crte_d between '2010-08-01' and '2010-08-31'
and (clnt_i = @ClientID or @ClientID = 0)
and (regn_i = @RegionID or @RegionID = 0)
0
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