Relationship
Given an ordered list of
m entities, E1, E2, ..., Em, a relationship R defines a rule of correspondence between the instances of these entities.
Specifically, R represents a set of m-tuples, a subset of the Cartesian product of entity instances, E1 × E2 × ... × Em.
- A relationship’s degree indicates the number of associated entities.
- Relationships are represented by diamond-shaped symbols ◇.
Lines are used to connect the entities to one another, passing through the diamond-shaped relationship symbol.
- Cardinality expresses the specific number of entity occurrences associated with one occurrence of the related entity.
One-to-One Relationship One-to-Many Relationship Many-to-Many Relationship 
min-card(E,R)=0
max-card(E,R)=1
min-card(F,R)=0
max-card(F,R)=1min-card(E,R)=0
max-card(E,R)=N
min-card(F,R)=1
max-card(F,R)=1
F is the “many” side here.min-card(E,R)=0
max-card(E,R)=N
min-card(F,R)=0
max-card(F,R)=N