We can suggest you two approaches which can be used independently the selected technology:
- Change the report parameter MultiValue property to true and handle the selection in the query using COALESCE for instance:
SELECT Job.*, Client.*
FROM Client INNER JOIN
Job ON Client.ClientID = Job.ClientID
WHERE COALESCE(@ClientID, NULL) IS NULL OR Job.ClientID in (@ClientID)
- Instead of using AllowNull add a value to the report parameter's AvalableValues.DataSource data. Thus you can interpret the additional value as required, and this interpretation will be the same for any viewer.
Let us know if you need any further help.
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