1. ## Rdbms notes

Chapter 3: Relational Model
Structure of Relational Databases
Relational Algebra
Tuple Relational Calculus
Domain Relational Calculus
Extended Relational-Algebra-Operations
Modification of the Database
Views
Example of a Relation
[IMG]file:///C:\Users\MANGESH\AppData\Local\Temp\msohtmlclip1\01\clip_image002.jpg[/IMG]
Attribute Types
Each attribute of a relation has a name
The set of allowed values for each attribute is called the domain of the attribute
Attribute values are (normally) required to be atomic, that is, indivisible
E.g. multivalued attribute values are not atomic
E.g. composite attribute values are not atomic
The special value null is a member of every domain

The null value causes complications in the definition of many operations
we shall ignore the effect of null values in our main presentation and consider
their effect later
Relation Schema
A1, A2, …, An are attributes
R = (A1, A2, …, An ) is a relation schema
E.g. Customer-schema =
(customer-name, customer-street, customer-city)

r(R) is a relation on the relation schema R
E.g. customer (Customer-schema)
Relation Instance
The current values (relation instance) of a relation are specified by a table
An element t of r is a tuple, represented by a row in a table

Database
n A database consists of multiple relations
n Information about an enterprise is broken up into parts, with each relation storing one part of the information

E.g.: account : stores information about accounts
depositor : stores information about which customer
owns which account
customer : stores information about customers

n Storing all information as a single relation such as
bank(account-number, balance, customer-name, ..)
results in

H repetition of information (e.g. two customers own an account)
H the need for null values (e.g. represent a customer without an account)

3. ## Re: Rdbms notes

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