ERD Design for Campground and Campsite I am designing erd for campground and campsite.
In general, a campground has many campsites in it. that lead to one to many relationships. but sometimes campsite has no campground, it appears as the single entity with no parent. then what relationship should be used for these?
This is quite normal in database design. If you look at this guide about cardinality notations you will see that one option is zero to one (optional) (one campsite has zero to one campgrounds).

In SQL you do it by making a FOREIGN KEY constraint from campsite to campground and not declaring the foreign key id in campsite as NOT NULL.