CardinalityWhat will be the maximum relationship between entities . This sometimes is also referred as mandatory part of relationship. Let’s clear this up with an example. If we talk about a relation between Student and University. I can say that Awais studied in a Government College University Lahore , now by rule Awais can be a student of only one university at max. Right? What about University? University can have one and more than one students. Relationship between Student-University , hence is one-to-many. What is the maximum relationship between two entities? It will either be one or many.
ModalityWhat will be the minimum relationship between entities. This is referred as optional part of relationship that entities can and cannot have . Taking another example of Awais’ University. Awais’s University gives out Annual Scholarships to its brightest and sharpest . Relationship between Student and Scholarship could be , any student may or may not have a Scholarship , but Scholarship will be given to at least one student . So Student may or may not have a Scholarship but Scholarship will have one or many students.
Reading Method :Student-University : Read the relationship from Student to University . Cardinality and Modality symbols will be on University side. One Student at max in One University and at least in One University. University-Student : One University can have more than one Student and at least one . Comment below the rest of the relationships , if you understand the concept here.
Relationships between entities are defined by cardinalities and modalities . Symbol at right side for maximum and left for minimum.