Since Crystal Reports 2008, designers have had the option to add optional parameters within their reports.
Optional parameters, provides designer the flexiblity to allow the end-user of the report to not have to select or enter a value for the parameter when the report is run.
In order for this to work correctly, Crystal Reports has introduced the HasValue function, to test to see if the Parameter field is empty
Effectively, this means that the parameter is being used in the report as the True condition, in the Select Criteria and will return all records for the field: (Not HasVAlue({?ParameterField}) or
{TableName.FieldName} = {?ParameterField})
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