Monash University 20041 Week 7 Data Modelling Relational Database Theory IMS1907 Database Systems.

Monash University 2004 1

Week 7

Data ModellingRelational Database Theory

IMS1907Database Systems


Detailed Data Modelling

The objective of detailed data modelling is to develop data structures that- have stability, minimum redundancy, and are flexible to

allow for future change- can be used as the basis for physical file and

database design- reflect the actual data requirements of the system


To expand the conceptual data model we need to identify and describe – the details of the entities and relationships– attributes (data elements) of entities and relationships

An organisation-wide perspective should be adopted to ensure minimum redundancy and inconsistency and to facilitate data sharing



Techniques- attribute collection- convert ER models to relations- normalisation- convert to data structure diagram (DSD)



At this stage our ER models are logical– independent of implementation technology

We shall consider converting our ER models using the relational data model because– it is the most commonly used in DBMS applications– some principles of logical database design for the

relational model apply to other logical models– high level of CASE tool support



Introduced by E.F. Codd in 1970

Represents data in the form of tables

Based on mathematical theory– solid theoretical foundation

Three components are considered– data structure– data manipulation– data integrity

Relational Data Model


Data structure– the relational database model represents data in the

form of tables or relations

Important concepts are- relation- primary key- composite key- foreign key

Relational Data Model


A relation is a named, two-dimensional table of data

Each relation consists of a set of named columns and an arbitrary number of unnamed rows

Each column corresponds to an attribute of the relation

Each row corresponds to an instance (or record) that contains values for an instance of that entity

Relations


A relation generally corresponds to some real world object or concept of interest to the system -similar to an entity

Emp# Name Salary Dept

1247

1982

9314

Adams

Smith

Jones

24000

27000

33000

Finance

MIS

Finance

Employee

Employee (Emp#, Name, Salary, Dept)

Relations


Relational tables are tables in which- each relation has a unique name- each attribute or column has a unique name- data values are atomic and single-valued- data values in columns are from the same domain- each row in the relation is unique- the sequence of columns is insignificant- the sequence of rows is insignificant

Properties of Relations


An attribute or group of attributes which uniquely identifies a row of a relation

A search criteria that would yield only one record

Entity integrity (relational data base theory) requires that each relation has a non-null primary key

Represented by underlining the key element– Employee (Emp#, Name, Salary, Dept)

Can be a composite key– Order-item (Order#, Item#, Qty-ordered)

Primary Key


Candidate keys– where several possible keys are identified, each of

which uniquely identifies the other attributes, they are known as candidate keys - choose one to be the primary key

– choose the most stable key or the key over which there is the most control

– treat the remaining candidates as non-key attributes• we can deal with them later if they give us trouble!

Primary Key


A foreign key is an attribute in one relation that is also a primary key in another relation in the same database– it may be a composite key– allows user to see records in one relation associated

with a record in another elation

The referential integrity constraint (relational database theory) specifies that if an attribute value exists in one relation then it must also exist in a linked relation

A foreign key must satisfy referential integrity

Foreign Key


In the example below, if a given Dept# exists in an Employee relation then that Dept# must exist in the Department relation

Employee (Emp#, Name, Salary, Dept#)

Department (Dept#, Dname, Budget)

foreign key

Foreign Key


The relational model has several constraints or rules that facilitate the maintenance of the accuracy and integrity of data in the database– domain constraints– entity integrity– referential integrity

Integrity Constraints


Domain constraints– all values appearing in a column of a relation must come

from the same domain of values

Domain definitions consist of – domain name– meaning– data type– size or length– allowable values or range of values



Entity integrity– no primary key attribute or component of primary key

attribute can be null– ensures every relation has a primary key and all data

values for that key are valid

Null value– allowable when value is unknown or is not applicable– not a value or a zero, rather the absence of a value– inclusion in relational model is controversial – can lead

to anomalies



Referential integrity– each foreign key value must match a primary key value

in another relation, or the foreign key value must be null– associations between tables are defined using foreign

keys– maintains consistency between rows in two relations– ensures every foreign key does correspond to an

existing primary key in another relation else the foreign key is null



Well-Structured Relations

A well-structured relation– is robust, stable and flexible– contains a minimum amount of redundancy– allows users to insert, modify, and delete rows in a table

without errors or inconsistencies • known as “anomalies”



Three types of anomaly are possible- insertion - deletion - modification

Relations that have been normalised to third normal form (3NF) are considered to be well structured relations– although even 3NF relations can have anomalies



Insertion anomaly– cannot create a new record without supplying data

values for for two independent data elements

Deletion anomaly– cannot delete a record without losing related data in that

record that might need to exist independently

Modification anomaly– must sometimes update a data value in many records to

ensure consistency


A Poorly-Structured RelationCan’t add new employee till completed course

Delete emp no 150 and lose Delphi

Name changes need to be made in more than one record

Emp_ID Emp_Name Course Date_Comp

100 Jim VB .Net 2/4/03

100 Jim C++ 6/8/03

140 Jane C++ 12/10/02

110 Wendy Java 30/6/03

110 Wendy VB .Net 2/4/03

150 Robert Delphi 1/5/04


Hoffer, J.A., Prescott, M.B. and McFadden, F.R., (2005), Modern Database Management, (7th edn.), Pearson Education Inc., Upper Saddle River, NJ, USA.

Ch. 5

References

Monash University 20041 Week 7 Data Modelling Relational Database Theory IMS1907 Database Systems.

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