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.