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DBMS RELATIONAL MODEL

 


DBMS RELATIONAL MODEL

Introduction:


E.F. Codd (Edgar Frank Codd) of IBM had written an article “A relational model for large shared data banks” in June 1970 in the Association of Computer Machinery (ACM) Journal, Communications of the ACM. His work triggered people to work in relational model. One of the most significant implementations of the relational model was “System R,” which was developed by IBM during the late 1970s. System R was intended as a “proof of concept” to show that relational database systems could really build and work efficiently. It gave rise to major developments such as a structured query language called SQL which has since become an ISO standard and de facto standard relational language. Various commercial relational DBMS products were developed during the 1980s such as DB2, SQL/DS, and Oracle. In relational data model the data are stored in the form of tables.

Relational Data Model


The relational model uses a collection of tables to represent both data and the relationships among those data. Tables are logical structures maintained by the database manager. The relational model is a combination of three components, such as Structural, Integrity, and Manipulative parts.

Structural Part

The structural part defines the database as a collection of relations.

Integrity Part

The database integrity is maintained in the relational model using primary and foreign keys.

Manipulative Part

The relational algebra and relational calculus are the tools used to manipulate data in the database. Thus relational model has a strong mathematical background. The key features of relational data model are as follows:

– Each row in the table is called tuple.
– Each column in the table is called attribute.
– The intersection of row with the column will have data value.
– In relational model rows can be in any order.
– In relational model attributes can be in any order.
– By definition, all rows in a relation are distinct. No two rows can be exactly the same.
– Relations must have a key. Keys can be a set of attributes.
– For each column of a table there is a set of possible values called its domain. The domain contains all         possible values that can appear under that column.
– Domain is the set of valid values for an attribute.
– Degree of the relation is the number of attributes (columns) in the relation.
– Cardinality of the relation is the number of tuples (rows) in the relation.



CLICK HERE FOR NOTES  RELATIONAL MODEL (UNIT-3)

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