Relationships
After two or more entities are identified and defined with attributes, the participants determine if a relationship exists between the entities. A relationship is any association, linkage, or connection between the entities of interest to the business; it is a two-directional, significant association between two entities, or between an entity and itself. Each relationship has a name, an optionality (optional or mandatory), and a degree (how many). A relationship is described in real terms.
Assigning a name, optionality, and a degree to a relationship helps confirm the validity of that relationship. If you cannot give a relationship all these things, then perhaps there really is no relationship at all.
Relationship represents an association between two or more entities. An example of a relationship would be:
• Employees are assigned to projects
• Projects have subtasks
• Departments manage one or more projects
Relationships are the connections and interactions between the entities instances e.g.
DEPT_EMP associates Department and Employee.
• A relationship type is an abstraction of a relationship i.e. a set of relationships instances sharing common attributes.
• Entities enrolled in a relationship are called its participants.
The participation of an entity in a relationship is total when all entities of that set might be participant in the relationship otherwise it is partial e.g. if every Part is supplied by a Supplier then the SUPP_PART relationship is total. If certain parts are available without a supplier than it is partial.
Naming Relationships:
If there is no proper name of the association in the system then participants’ names of abbreviations are used. STUDENT and CLASS have ENROLL relationship. However, it can also be named as STD_CLS.
Roles:
Entity set of a relationship need not be distinct. For example
The labels “manager” and “worker” are called “roles”. They specify how employee entities interact via the “works-for” relationship set. Roles are indicated in ER diagrams by labeling the lines that connect diamonds to rectangles. Roles are optional. They clarify semantics of a relationship.
Symbol for Relationships:
• Shown as a Diamond
• Diamond is doubled if one of the participant is dependent on the other
• Participants are connected by continuous lines, labeled to indicate cardinality.
• In partial relationships roles (if identifiable) are written on the line connecting the partially participating entity rectangle to the relationship diamond.
• Total participation is indicated by double lines
Types of Relationships
o Unary Relationship
An ENTITY TYPE linked with itself, also called recursive relationship. Example Roommate, where STUDENT is linked with STUDENT
Example 1:
Roommate
Example 2:
Sponsored
o Binary relationship
A Binary relationship is the one that links two entities sets e.g. STUDENT-CLASS. Relationships can be formally described in an ordered pair form.
Enroll = {(S1001, ART103A), (S1020, CS201A), (S1002, CSC201A)}
Entire set is relationship set and each ordered pair is an instance of the relationship.
o Ternary Relationship
A Ternary relationship is the one that involves three entities e.g.
STUDENT-CLASS-FACULTY.
o N-ary Relationship
Most relationships in data model are binary or at most ternary but we could define a relationship set linking any number of entity sets i.e. n-ary relationship
Entity sets involved in a relationship set need not be distinct. E.g.
Roommate = {(Student1, Student2) | Student1 ∈ Student Entity Set, Student2 ∈ Student Entity Set and Student 1 is the Roommate of Student2}
Relationship Cardinalities
The cardinality of a relationship is the number of entities to which another entity can map under that relationship. Symbols for maximum and minimum cardinalities are:
o One-to-One mapping:
A mapping R from X to Y is one-to-one if each entity in X is associated with at most one entity in Y and vice versa.
o Many-to-One mapping:
A mapping R from X to Y is many-to-one if each entity in X is associated with at most one entity in Y but each entity in Y is associated with many entities in X.
o One-to-Many mapping:
A mapping R from X to Y is one-to-many if each entity in X is associated with many entities in Y but each entity in Y is associated with one entity in X.
o Many-to-Many mapping:
A mapping R from X to Y is many-to-many if each entity from X is associated with many entities in Y and one entity in Y is associated with many entities in X.
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