Definition of a Relationship
A relationship is an association among two or more entities or other relationships. Relationships are expressed as verbs.
Relationships among entities are described by one of three ratios:
|
Relationship |
Shorthand Notation |
|
One-to-one |
1:1 |
|
One-to-many |
1:M |
|
Many-to-many |
M:M |
Relationships as defined in the E-R model have no direct counterpart in relational theory. The closest property of relational theory to expressing what a relationship is in E-R theory is the primary key-foreign key relationship.
“Relationship Theory” on page 69, “One-to-One Relationships” on page 70, “One-to-Many Relationships” on page 71, and “Many-to-Many Relationships” on page 72 describe the properties of relationships in greater detail.