Week 3
3.3 Relationships and Cardinality Ratios
A RELATIONSHIP TYPE represents an association between two or more entity types. In ER diagrams, relationships are drawn as DIAMONDS connected to the participating entity types.
RELATIONSHIP EXAMPLES from Newgate University:
1. STUDENT ENROLS-IN COURSE
2. LECTURER TEACHES COURSE
3. STUDENT BELONGS-TO DEPARTMENT
4. STAFF HAS DEPENDENT
Cardinality Ratios
Cardinality describes the maximum number of times an entity instance in one entity type can be associated with instances of another entity type.
|
Cardinality |
Notation |
Nigerian Example |
|
One-to-One (1:1) |
1 — 1 |
One STAFF member has exactly one NIN (National Identity Number) |
|
One-to-Many (1:N) |
1 — N |
One DEPARTMENT has many STUDENTS; each STUDENT belongs to one DEPARTMENT |
|
Many-to-Many (M:N) |
M — N |
Many STUDENTS enrol in many COURSES; each COURSE has many STUDENTS |