It is a set of database, which consist of some relational pattern and may set of constraints on relation. Sets can be defined as collections of same type of objects, having no order and no duplicacy. Implied ordering found in relative attributes but it could be used as functions and take reference by name.
Relationship between tables is shown by primary and foreign keys. It is a set of attributes in which each and everyone have unique identity in relations.
It is main key of the relation, each relation have one. Primary key must not have null value. It is unique value of each tables. Such as
It is a set of attributes of primary key of other tables which make relation between two tables, that means a foreign key of table B is match primary key of table A. Here not necessary that the attribute name should be same but must have same domain.