Some important Terminologies in Relational Algebra :

1. Attribute : Attributes are the characteristics that describe entities and relationships. For example, an entity Employee is described by attributes including: Employee_id, Employee_name, Account_branch etc.

2. Relation Schema : A relation schema represents name of the relation with its attributes. For example Employee(Employee_id, Employee_name, Account_branch) is a relation schema for Employee. If a schema has more than one relation, it is called Relational Schema.

3. Tuple :  Each row in the relation (or table) is known as tuple. For example the above relation contains 5 tuples, one of which is shown as:
  E001  John  Axis_Bank
4. Relation instance : The set of tuples of a relation at a particular instance of time is called as relation instance. The above table shows the relation instance of Employee at a particular time. It can change whenever there is insertion, deletion or updation in the database.

5. Degree : The number of Attributes in the relation is known as degree of the relation. The Employee relation defined above has degree 3.

6. Cardinality : The number of tuples in a relation is known as cardinality. The Employee relation defined above has cardinality 5.



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