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Relational Model  

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Introduction

The Relational Database

Management System (RDBMS) has

become the dominant DBMS in use

today.

All data are logically structured

withing relations (tables).

Based on the relational data model

proposedby Dr E.F . Codd.

Design methology will be based on the

relational model

What is a data model ? Data model : Integrated collection of concepts

for describing data (data requirements).

A data model may comprise 3 componants :

A structural part ( rules on how database to be

constructed)

Manipulative part (Type of operations allowed

on the data)

Set of integrity rules ( to ensure data accuracy)

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TerminilogyIn the relational model, we use relations ( tables ) to hold info about the objects that we want to represent in the database.

Relation : Table with column and rows

Attribute : named column of a relation

Domain: set of allowable (possible)values for one or more

attributes.

Tuple: a record of a relation (row in the table)

Relational Database - collection of normalized relations with

distinct relation names.04/21/23

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Examples of domain for attributtes

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Properties of Relations Table name is distinct from all other table names

in the database. Each cell of table contains exactly one atomic

(single) value. Each column has a distinct name. Values of a column are all from the same domain. Each record is distinct (no duplicate record) Order of columns has no significance. Order of records has no significance, theoretically

(affect efficiency of accessing records )

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Relational Keys

Superkey A column, or a set of columns (minimum

number), that uniquely identifies a record within a table.

Candidate Key “K” Uniqueness : value uniquely identify that

record . Irreducibility : No proper subset of K

has the uniqueness property

Each record in a table must be unique , so we need a mean to provide this uniqueness.

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“city” cannot be selected as a candidate key for the Branch table . Why?What about “zipCode”?

“Branch” table

Composite keyRole table represents the name of the character played by an actor in videos.

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Relational Keys Primary Key

Candidate key selected to identify records uniquely within

table.

Alternate Keys

Candidate keys that are not selected to be primary key.

Example : for Branch table , if “branchNo” is the primary

key then “zipCode” is the alternate key.

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Relational Keys Foreign Key

Column, or set of columns, within one table that

matches candidate key of some (possibly same) table.

Note:

When a column appears in more than one

table , its appearance usually represents a

relationship between records of the two

tables

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Relational Integrity

Every column has an associated domain (constraint

the value that a column can take).

There are 2 more important integrity rules that apply

to all instances of the database:

Entity integrity

Referential integrity

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Relational Integrity

Null Represents value for a column that is currently

unknown or not applicable for record.

Deals with incomplete or exceptional data.

Represents the absence of a value and is not

the same as zero or spaces, which are values.

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Relational Integrity Entity Integrity

In a base table, no column of a primary key can

be null.

Referential Integrity

If FK exists in a table, either FK value must

match a candidate key value of some record in

its home table or FK value must be wholly null.

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Relational Integrity Business Rules

Rules that define or constrain some aspect of the organization.

Example : a member can only rent a maximum of 10 videos at any one time.

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Relational Languages Two main languages:

SQL (Structured Query Language), standardized by ISO.

QBE (Query-by-Example), alternative graphical “point-and-click” way of querying database.

Schema Diagram

Relational Database Keys

Table 2.3

Figure 2.4 An Illustration of Integrity Rules

An Example Table

Students (sid: string, name: string, login: string, age: integer, gpa: real)

sid name login age gpa

50000 Dave dave@cs 19 3.3

53666 Jones jones@cs 18 3.4

53688 Smith smith@ee 18 3.2

53650 Smith smith@math 19 3.8

53831 Madayan madayan@music 11 1.8

53832 Guldu guldu@music 12 2.0

Another example: Courses

Courses (cid, instructor, quarter, dept)

cid instructor quarter dept

Carnatic101 Jane Fall 06 Music

Reggae203 Bob Summer 06 Music

Topology101 Mary Spring 06 Math

History105 Alice Fall 06 History

Keys

Primary key – minimal subset of fields that is unique identifier for a tuple sid is primary key for Students cid is primary key for Courses

Foreign key –connections between tables Courses (cid, instructor, quarter, dept) Students (sid, name, login, age, gpa) How do we express which students take each

course?