RDBMS and Relational Algebra

RDBMS and Relational Algebra

Concept of RDBMS

            A relational database consists of a collection of tables or relations. Relation is a mathematical tern for a table.  Each table represents a relationship among a set of values. Since a table is a collection of such relationships it is called as a relation. The figure below represents a STUDENT relation.

Adm No	Roll No	Year	Name	Batch	Father	Mother	Dist code
22002	1	2011	Jibin	BA	Babu	Seetha	D-01
22003	20	2000	Deepu	BCOM	Santhosh	Sandhya	D-09
22004	13	2008	Vipin	BBA	Rajan	Rejani	D-02
22005	14	2012	Vyshakh	BA	Kiran	Kalyani	D-03
22006	19	2004	Prajeesh	BSC	Jayan	Jayalalitha	D-07
22007	21	2009	Dipin	BSC	Prakash	Prajitha	D-06
22008	9	2008	Jijesh	BCOM	Mohan	Latha	D-04

Domain: A domain is a pool of values from which actual values appearing in a given column are drawn.  The year column in STUDENT relation can have any valid year as its domain.

Tuple:  The rows of a relation are generally referred to as tuples. Each row in the STUDENT relation represents the personal details of a particular student.

Attributes: The columns of a relation are called attributes.  RegNo, RollNo, Year, Name, Father, and Mother are attributes of the STUDENT relation.

Cardinality:  The number of rows or tuples in a relation is called cardinality of the relation. The relation STUDENT has seven tuples and hence the cardinality of STUDENT relation is 7.

Degree: The number of attributes in a relation determines the degree of a relation. The relation STUDENT has 8 columns or attributes and therefore the degree of STUDENT relation is 8

Keys

Primary key: A primary key is one of the candidate keys chosen to be the unique identifier for a table by the database designer. But a super key is a combination of a primary key with any other attribute or group of attributes.

  

Alternate key: The candidate key that is not the primary key is called alternate key. A single attribute or a set of attributes which is a candidate key in another table is called foreign key. Foreign key helps to maintain integrity of the database.