02 Database Design ER Model

7/31/2019 02 Database Design ER Model

1/22

Computer Science, University of Brawijaya

Putra Pandu Adikara, S.Kom

Database Design & ER DiagramsBasis Data 2


2/22

ER, Relational Model


3/22

Database Design

The database design process can be dividedinto six steps. The ER model is most relevant tothe first three steps.

1. Requirements Analysis2. Conceptual Database Design

3. Logical Database Design


4/22

The remaining three steps of database design

4. Schema Refinement

5. Physical Database Design6. Application and Security Design


5/22

Entities, Attributes, And Entity Sets

An entity is an object in the real world that isdistinguishable from other objectsIt is often useful to identify a collection of similar entities.

Such a collection is called an entity setAn entity is described using a set of attributes . Allentities in a given entity set have the same attributes; thisis what we mean by similar.

For each attribute associated with an entity set, we must identifya domain of possible valuesderived attribute can be obtained from other attributes or relatedentities

Weak and Strong Entity is


6/22

Relationships And Relationship Sets

A relationship is an association among two or moreentities.

Set of similar relationships is called relationshipset . A relationship set can be thought of as a set ofn-tuples:

A relationship can also have descriptive attributes used to record information about the relationship


7/22

Additional Features Of The ER Model

Key Constraints statement that a certain minimal subset of the fields of a relation is a unique identifier for atuple

A set of fields that uniquely identifies a tuple according toa key constraint is called a candidate key for therelation; we often abbreviate this to just key Sometimes the information stored in a relation is linkedto the information stored in another relation thats why weneed Foreign Key.

The foreign key in the referencing relation must match theprimary key of the referenced relation, that is must have thesame number of columns and compatible data types


8/22

Cardinality Constraints

Cardinality of relationshipOne to oneOne to many

Many to many


9/22

Participation Constraints


10/22


11/22

Additional Features Of The ER Model

Weak EntitiesClass Hierarchies


12/22

Enhanced ER Models (EER)

Semantic modelling concepts (abstraction)SpecialisationGeneralisation

Classification/CategorisationAggregation

First we need some new entity constructs.Superclass - an entity type that includes distinct

subclasses that require to be represented in a data model.Subclass - an entity type that has a distinct role and is alsoa member of a superclass.


13/22

Specialisation

This is the process of maximising the differences between membersof an entity by identifying their distinguishing characteristics.

Here we have shown that the manages relationship is only applicable to theManager subclass, whereas the works_for relationship is applicable to allstaff.It is possible to have subclasses of subclasses.


14/22

Generalisation

Generalisation is the process of minimising thedifferences between entities by identifying commonfeatures.

This is the identification of a generalised superclass fromthe original subclasses. This is the process of identifyingthe common attributes and relationships.For instance, taking:

car(regno,colour,make,model,numSeats)motorbike(regno,colour,make,model,hasWindshield)

And forming:vehicle(regno,colour,make,model,numSeats,hasWindshielf)


15/22


16/22


17/22

Integrity Constraints Over Relations

Key ConstraintsWhich one is the primary key used in the table

Foreign Key ConstraintsWhich one is the foreign key to refer to another table

General Constraints

The domain in the attribute


18/22

Enforcing Integrity Constraints

Insert if not exists in referenced table, reject theinsertionDelete can be one of the options:

Delete all rows in referenced tableDisallow deletionSet the primary key to the existing default rowSet the primary key to null

Update same as previous


19/22

Enforcing Integrity Constraints

CREATE TABLE Enrolled (

studid CHAR(20) ,cid CHAR(20) ,

grade CHAR(10),

PRIMARY KEY (studid, dd),

FOREIGN KEY (studid) REFERENCES Students

ON DELETE CASCADE

ON UPDATE NO ACTION)

CASCADE , if a Students row is deleted, all rows that refer to it areto be deleted as wellON DELETE SET DEFAULT set to default as defined in defaultvalue constraint


20/22


21/22

Normalization


22/22

Schema Refinement

Why do we need decomposition?What is Functional Dependency (FD)?Normal forms

1NF2NF3NFBoyce-Codd (BCNF)

4NF5NF

02 Database Design ER Model

Documents

Transcript of 02 Database Design ER Model