How to pass optional date parameters in KSH script

I think this should be simple, but I keep running into problems. I simply want to return all data from a table that lies between a date range. But I want the date range to be optional.

1. Monkeyfile.ksh the query will look for the last run date as start date (based on a banana log table), current date or sysdate as end date

2 Monkeyfile.ksh 'dd-mon-yy' query will create files based on one date passed (start date) query takes ‘dd-mon-yy’(the date passed) as begin date, current date as end date

3 monkeyfile.ksh ‘dd-mon-yy’ ‘dd-mon-yy’ - 2 dates are passed, first is begin date, 2nd is end date

How do I write a if statement for this in ksh