Mapping cardinalities, or
cardinality ratios, express the number of entities to which another
entity can be associated with via a
relationship set. They are most useful in describing
binary relationship sets, however, they can be used to describe relationships sets involving more than two entity sets.
One-to-One: An entity in A is associated with, at most, one entity in B, and an entity in B is associated with, at most, one entity in A.
a1 - b1
a2 - b2
a3 - b3
One-to-Many: An entity in A is associated with any number of entities in B. An entity in B, however, can be associated with, at most, one entity in A.
a1 - b1, b2
a2 - b3, b4, b5
a3 - b6
Many-to-One: An entity in A is associated with, at most, one entity in B. An entity in B can be associated with any number of entities in A.
a1 - b1
a2, a3 - b2
a4, a5, a6 - b3
Many-to-Many: An entity in A can be associated with any number of entities in B. An entity in B can be associated with any number of entities in A.
a1 - b1
a2, a3 - b2, b3
a4, a5, a6 - b4, b5
SOURCE: Database System Concepts, Abraham Silberschatz, Henry F. Korth, S. Sudarshan.