Association between two or more entities where each has (or plays) a specific role.
In SQL, a relationship is represented by sets of n-tuple records in a table (the relation predicate).
In this kind of relationship, each record represents a collection of tuples that describes an entity.
Basically, each entity is identified by a primary key derived from the value of a
designated table column. Outside the table of original designation, a primary-key
value is referred to as a foreign-key when used to construct records in other tables where
it doesn't function as the primary-key, i.e., those tables use this as a reference mechanism
for identifying records in other tables.