Relationship and Relationship set
A relationship is an association among several entities.
A relationship set is a set of relationship of the same type
Let E1, E2, ... En be a set of entity sets.
<e1, e2, ..., en> is a relationship, where ei is contained in Ei.
a subset of E1 x E2 x ... x En is a relationship set
We say E1, E2, ... En participate in the relationship R
Ei plays a role in the relationship R
If E1, E2, ... En are not distinct, the different roles of their participation must be specified