Key Concepts of The Relational Data Model and Relational Model Constraints
In this article, we shall consider key concepts of the relational data model and relational database constraints.
First, let us consider terms in the relational data model.
Terms in the Relational Data Model
- Relational model: a collection of relations.
- Relation: table
- Tuple: row
- Attribute: column
- Domain: a set of atomic values
- Atomic values: single values
- Degree of a relation: number of attributes in a relation
- Cardinality: number of tuples in a relation.
- Relation schema: describes the relation by specifying relation name, list of attributes, and degree of the relation.
For example, a relation schema for student:
student(id, name, phone, address, age, gpa)
student is a relation name. It has six attributes.
10. Relation state: relational instance at a particular moment in time (a set of tuples)
11. Relational database schema: contains a set of relational schemas and integrity constraints such as entity integrity constraint, referential integrity constraint, key and domain constraint.
Data Definition Language (DDL) helps us to describe the relational database schema.
12. Relational database state: a set of relational schema states at a particular moment in time.
13. Relational model: represents a collection of relational schemas and constraints.
Tuples in a relation are not ordered
A relation is a set of tuples. Set has no orders. Therefore tuples in a relation are not ordered. It will be identical: attributes and values are maintained. Relation represents data at a logical level.
Duplicate tuples are not allowed in a relation
Because it violates the key constraint. We can not identify each tuple uniquely in a relation if we have duplicate tuples.