Relationships and Relationship Types

1. Relationships

Relationship: The association among entities is called a relationship.

2. Relationship Type

Relationship type: Defines a relationship set among entities of certain entity types. We assign a name to a relationship type (usually) using a verb.

An entity type can have more than one relationship with another entity type. Relationship types may be given role names to indicate purpose that each participating entity type plays in a relationship.

3. Degree of a Relationship

Degree of a Relationship: The degree of a relationship is the number of entity types participating in the relationship. 

When we speak of a student registered for a course, we are discussing a relationship, register, where two entity types (Student and Course) are involved; the relationship is of degree 2 because each instance of register will always involve one student entity type and one course entity type.

Relationships of degree 2 are called binary relationships; relationships of degree 3 are called ternary relationships[三元关系]. In general, we speak of n-ary relationships where n entities participate in a relationship.

3.1 Recursive Relationship

A Recursive Relationship[递归关系] is one in which a relationship exists between occurrences of the same entity type. These are also called unary relationships[一元关系].

A typical business example arises when we have a business rule such as "a staff member supervises other staff members". • The role of one staff member may be supervisor[管理者] whereas the role of another staff member may be supervisee.

4. Structural Constraints

4.1 Cardinality

Cardinality: Cardinality[基数] is a constraint[限制] on a relationship specifying the number of entity instances that a specific entity may be related to via the relationship.

There are three classifications, and these are:

• one-to-one (1:1)

• one-to-many (1:*)

• many-to-many (*:*)

4.1.1 one-to-one (1:1) Cardinality

One-to-one relationships have 1 specified for both cardinalities, and do not seem to arise very often. To illustrate a one-to-one, we require very specific business rules.

Suppose we have Staff and Vehicles. Assume that we are only concerned with the current driver of a vehicle, and that we are only concerned with the current vehicle that a driver is operating. Then, we have a one-to-one relationship between Staff and Vehicle.

4.1.2 one-to-many (1:*) Cardinality

For example, A Branch entity has many Staff members; and each Staff entity works in one Branch

4.1.3 many-to-many (*:*) Cardinality

For example, An Actor entity stars in many Videos; and a Video entity has many Actors.

4.2 Participation

Participation: Determines whether all or only some entity occurrences participate in a relationship.

There are 2 classifications for participation, these are optional and mandatory[强制].

• Optional means that each entity may or may not participate in the relationship.

• Mandatory means that each entity must participate in the relationship

To represent (record) the cardinality and participation constraints, we will use 2 numbers (min, max) where min represents the minimum participation of an entity in the relationship (participation). Max represents the maximum participation of an entity in the relationship (cardinality).

• The min value can be 0 (for optional) or 1 (for mandatory).

• The max value can be 1 (for 1) or * (for many).

• Note that these values may vary if a specific min and/or max value is given

4.3 Summary

5. Relationship Type Attributes

Attributes[属性] can also be assigned to relationship types. They represent a data item value that needs to be recorded when an entity occurrence on each side of the relationship is associated with one another.

• For Example, suppose we have entity types Student and Module that are associated via a studies relationship type. An attribute that helps to describe a Student studying a Module is grade. This will record the grade that each student obtains on each module that they are associated with.