Ø Relational model is an attempt to simplify the database structure.
Ø It represents all data in the database as simple tables in the row-column format.
Ø RDBMS can be defined as a DBMS where all data visible to the user is organised strictly as tables of data values and where all database operations work on these tables.
Relational Data Structure
Ø The organizing principle in a relational database is the table, a tabular arrangement of data values:
v A table is called a relation.
v The row (or record) in the table is called a tuple.
v The column (or field) is called an attribute.
v The number of Tuples is called the cardinality, and the number of attributes is called the degree of the table.
Ø A domain is a pool of values from which one or more attributes (columns) draw their actual values.
Ø Missing or unknown information is represented as NULL in a relational system:
Ø NULL is not the same as space or zero.
Ø Attributes are a property of a given entity.
Ø Attributes are depicted as ellipses, labeled with the name of the property.
Subtypes and Supertypes
v Is a subset of another entity. It is always dependent on the supertype for its existence
v Is connected to the supertype by an unnamed relationship
v Is connected to the relationship with a line containing a crossbar
Ø Relational Operators are of the following types:
v RESTRICT: Extracts specified tuples or rows from a given relation, based on a condition.
v PROJECT: Extracts specified attributes or columns from a given relation.
v PRODUCT: Builds a relation from two specified relations. It consists of all possible combinations of tuples, one from each of the two relations.
v UNION: Builds a relation from Tuples Appearing in either or both of the specified relations. To be union compatible, the two tables should have the same types of attributes.
v INTERSECT: Builds a relation consisting of tuples that appear in both relations.
v DIFFERENCE: Builds a relation of tuples appearing in the first but not the second of two specified relations.
v JOIN: Builds a relation from two specified relations which consists of all possible combinations of tuples, one from each relation that satisfy the specified condition.
v DIVIDE: The Divide operator takes two relations and builds another relation consisting of values of an attribute of one relation that match all the values in the other relation.
Ø Several RDBMS products are available today. Some popular products are:
v Microsoft SQL Server
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