Lecture 3: Basic Database Analysis

ISOM3260, Spring 2014

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Where we are now• Database environment

– introduction to database• Database development process

– steps to develop a database• Conceptual data modeling

– entity-relationship (ER) diagram; enhanced ER• Logical database design

– transforming ER diagram into relations; normalization• Physical database design

– technical specifications of the database• Database implementation

– Structured Query Language (SQL), Advanced SQL• Advanced topics

– data and database administration

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Database development activities during SDLC

Top-downanalysis

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Basic Database Analysis

• Business Rules• E-R Model• Entities• Attributes• Relationships

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Business Rules

• Statements that define or constrain some aspect of the business

• Include– names and definitions of entity types, attributes, and

relationships– constraints on above data objects

• Expressed in terms familiar to end users– e.g. Every student in the university must have a faculty

advisor• Automated through DBMS software• Some business rules cannot be represented in E-R

diagram; instead they are stated in natural language

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Figure 2-1: Sample E-R Diagram

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Figure 2-2: Basic E-R Notation

Entity symbols

Relationship symbols

Attribute symbols

A special entity that is also a relationship

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E-R Model Constructs• Entity - person, place, object, event,

concept• Entity Type versus Entity Instance

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What Should an Entity Be?

• SHOULD BE:– an object that will have many instances in the

database– an object that will be composed of multiple attributes– an object that we are trying to model

• SHOULD NOT BE:– a user of the database system – an output of the database system (e.g. a report)

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Inappropriate entities

System user System output

Appropriate entities

Figure 2-4

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Strong vs. Weak Entity Type

• Strong entity – exists independently of other types of entities– has its own unique identifier– represented with single-line rectangle

• Weak entity– dependent on a strong entity…cannot exist on its own– does not have a unique identifier– represented with double-line rectangle

• Identifying relationship– links strong entities to weak entities– represented with double line diamond

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Figure 2-5: Strong and weak entities

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Attributes

• Attribute– property or characteristic of an entity type

• Classifications of attributes– Simple versus Composite Attribute– Single-Valued versus Multivalued Attribute– Stored versus Derived Attributes– Identifier Attributes

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Figure 2-7: A composite attribute

An attribute broken into component parts

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Figure 2-8: Entity with a multivalued attribute (Skill) and derived attribute (Years_Employed)

Derived from date employed and current date

Multivalued: an employee can have more than one skill

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Identifier Attribute

• Identifier (or key)– an attribute (or combination of attributes) that

uniquely identifies individual instances of an entity type

• Simple Key versus Composite Key• Candidate Key

– an attribute that could be a key…satisfies the requirements for being a key

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Characteristics of Identifiers

• Will not change in value• Will not be null• No intelligent identifiers

– e.g. containing locations that might change• Substitute new, simple keys for long, composite keys

– e.g. Game_Number instead of Home_Team and Visiting_Team

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Figure 2-9a: Simple key attribute

The key is underlined

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Figure 2-9b: Composite key attribute

The key is composed of two subparts

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Relationships

• Relationship Types vs. Relationship Instances– the relationship type is modeled as the diamond

and lines between entity types…the instance is between specific entity instances

• Relationships can have attributes– these describe features pertaining to the

association between the entities in the relationship

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Figure 2-10a: Relationship type (Completes)

Figure 2-10b: Relationship instances

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Figure 2-11a: A binary relationship with an attribute

Here, the Date_Completed attribute refers to the employee’s completion of a course…it is an attribute of the relationship

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Associative Entities• It’s an entity – it has attributes• AND it’s a relationship – it links entities together

• When should a relationship with attributes instead be an

associative entity? – all relationships for the associative entity should be many– the associative entity could have meaning independent of the other

entities– the associative entity preferably has a unique identifier, and should

also have other attributes– the associative entity may be participating in other relationships other

than the entities of the associated relationship– ternary relationships should be converted to associative entities

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Figure 2-11b: An associative entity (CERTIFICATE)

Associative entity involves a rectangle with a diamond inside.Note that the many-to-many cardinality symbols face toward the associative entity and not toward the other entities.

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Degree of a Relationship

• Degree of a Relationship is the number of entity types that participate in it– Unary Relationship (degree 1)– Binary Relationship (degree 2)– Ternary Relationship (degree 3)

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Figure 2-2: Degree of relationships

One entity related to another of the same entity type

Entities of two different types related to each other

Entities of three different types related to each other

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Figure 2-12a: Unary relationships

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Representing a bill-of -materials structure

Figure 2-13a: A unary relationship with an attribute. This has a many-to-many relationship

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(b) Two instances

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Figure 2-13c: An associative entity – bill of materials structure

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Figure 2-12b: Binary relationships

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Figure 2-12c: Ternary relationships

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Figure 2-14: Ternary relationships as an associative entity

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Cardinality of Relationships

• One–to–One– each entity instance in the relationship will have

exactly one related entity instance• One–to–Many

– an entity instance on one side of the relationship can have many related entity instances, but an entity instance on the other side will have a maximum of one related entity instance

• Many–to–Many– entity instances on both sides of the relationship can

have many related entity instances on the other side

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Cardinality Constraints

• Cardinality Constraints– the number of instances of one entity that can or must

be associated with each instance of another entity

• Minimum Cardinality– if zero, then optional– if one or more, then mandatory

• Maximum Cardinality– the maximum number

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Figure 2-2: Cardinality

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Figure 2-16a: Basic relationship with only maximum cardinalities showing

Figure 2-17a: Mandatory cardinalities

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Figure 2-17b: One optional, one mandatory cardinality

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Figure 2-17c: Optional cardinalities

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Figure 2-18: Cardinality constraints in a ternary relationship

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More about Relationships

• Modeling Time-Dependent Data– Time Stamps (such as date and time)

• Multiple Relationships– more than one relationship between the same

entity types

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Figure 2-19: Example of time stamping

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Examples of multiple relationships – entities can be related to one another in more than one way

Figure 2-21a: Employees and departments

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Figure 2-21b: Professors and courses (fixed upon constraint)

Maximum cardinality constraint is 4

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Figure 2-22: E-R diagram for Pine Valley Furniture