If you're looking for changes in numbers over time for a period of consecutive months, I would look at using a cross-tab. You then just filter for start date and end date.
If you're looking to compare two specified months, here's a thought on how to do this:
1. You'll need two copies of the table that is filtered by month - when you select it a second time in the Database Expert, Crystal will alert you that the table is already in the report and ask if you want to alias it. When you click on Yes (OK?) the table will appear in the selected tables list with "_1" on the end of the table name. For this example, I will call these "DateTable" and "DateTable_1".
2. If you have a master table that contains data that is common to both sets, left join both copies of the table to that. If you don't, left join from DateTable to DateTable_1 - in this case you won't be able to see any data that's in DateTable_1 that doesn't have a corresponding record in DateTable.
3. Create two parameters - I'll call them {?Month1} and {?Month2}
4. In the Select Expert filter on the date field from DataTable = {?Month1} and the date field from DataTable_1 = {?Month2}
5. If you have a master table, group on some field in the master table.
This should get you started. Good luck!
-Dell
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