Cardinality
defines the number of entities relationship with other number of entities.
Relationship set of n:m entities

Cardinality of 
Students to Subject is 1 :N
Student to Sports 1:N
Teachers to Subjects 1:1
Teachers to Students 1:N
Teachers to Sports 1:1
Coaches to Students N : M

Relationship Sets

One-to-one 
One entity from entity set A can be associated with at most one entity of entity set B and vice versa.

One-to-many
One entity from entity set A can be associated with more than one entities of entity set B But an entity from entity set B, can be associated with at most one entity


Many-to-one 
More than one entities from entity set A can be associated with at most one entity of entity set B, an entity from entity set B can be associated with more than one entity from entity set A. 


Many-to-many
One entity from A can be associated with more than one entity from B and vice versa.


when we design SQL databases, we introduce another relationship entity
between A and B to keep relationships between entities.
A > AB < B  creating 1:m:1 relationship

Participation Constraints

• Total Participation − Each entity is involved in the relationship. Total participation is represented by double lines.

• Partial participation − Not all entities are involved in the relationship. Partial participation is represented by single lines.
  
  Most business databases are designed with Partial participation .