What are the dates of the projects with at least 2 documents, please? | Do you mean the projects of documents, not projects, right? | Yes
select document_date from documents where project_id in  ( SELECT project_id FROM Documents GROUP BY project_id HAVING count ( * )    > =    3 ) 
dates of the projects with at least 2 documents refers to document_date from documents where project_id has a count of documents >= 2;