Avoid recursive relationships where possible. Avoid relationships including more than two entity types where possible. Be alert for fan-traps and chasm traps. If two entities are related then you should be able to read across the diagram and tell unambiguously which is related to which. Y, who the pilot is, which specific plane is in use, who the flight attendants are, and so on.
Participation Constraints Use a double-line to indicate a mandatory relationship .
If a relationship type is mandatory for an entity type then each entity must take part in a relationship. For example, it may be mandatory for a supervisee to have a supervisor. Participation constraints can be combined with cardinality by using the Mini-Max notation. If the minimum is more than 0 then participation is mandatory. Write cardinality between the relationship diamond and the line leading to each entity type.