I've tried to do an inner join SOQL query using Events/Tasks:
SELECT Id FROM Attachment WHERE ParentId IN (SELECT Id FROM Event) LIMIT 1000
But I failed with the error:
"Entity 'Event' is not supported for semi join inner selects"
I was wondering which are the not supported entities; I tried to figure out at Salesforce documentation but I wasn't lucky at all.
Does anyone of you have this valuable information or knows where to find out?