I have two reports "Lead history" and "Lead Activity history" which I want to join using joined reports. If I try to join theses reports,I am unable to find common fields between them as a result I get the error "Incompatible Report Type
This report type cannot be used, because it has no fields in common with the report types in your joined report.".
How to implement this
You can use custom report types and add there if you want the relationship object A with or without records related to object B. then you don't need to create a filter anymore.
Sven
Please refer: How Joined Reports Work
A field is a common field if it’s shared by all report types or if all report types share a lookup relationship to the field. These fields appear in the Common Fields area in the Fields pane, and can be used to group report blocks.
Best Answer
This isn't going to be possible the way you defined it
Hence, no common object to join
Because there is no common object, but a conceptual common
Lead.Id, you might be better off writing a simple APEX VF controller and VF page to do the joins in memory into a single wrapper class. You'll need to think about what your layout is going to look like as for a given lead, there are n field histories and m activities