Chapter 4: Data Modeling & the Entity-Relationship Model

How are the three levels of Entities portrayed, in a data model?

1.) Entity w/ All Attributes 2.) Entity w/ Identifier Attribute Only 3.) Entity w/ No Attribues

Identifiers are similar to keys in a relational model, but what are two important differences?

1.) They are a logical concept: consisting of one or more attributes that users think of as a name of the entity. They may or may not be represented as keys in the database design. 2.) Primary and Candidate keys must be unique, whereas identifiers don't have to be.

What is an Attribute?

A description of an entity's characteristics. Ex.) Emplyee Name, Date Of Hire, Job Skill Code

What do Use Cases consist of?

A description of the roles uses will play when utilizing the new system, together with descriptions of activities' scenarios.

Entity-Relationship Diagram (E-R Diagram)

A graphic used to represent entities and their relationships.

What are HAS-A Relationships

A relationship between two entities or objects that are of different logical types.

What is a Non-Identifying Relationship?

A relationship between two entities such that one is not ID-dependent on the other.

What is a Recursive Relationship?

A relationship in which an entity has a relationship with itself.

What is a IS-A Relationship?

A relationship that connects supertypes and subtypes.

What is an Identifying Relationship?

A relationship that is used when the child entity is ID-dependent upon the parent entity.

What is a Data Model?

A representation of the content, relationships, and constraints of the data needed to support the requirements.

What is Unified Modeling Language (UML)?

A set of structures and techniques for modeling and designing object-oriented programs and applications. It is a methodology and a set of tools for such development and incorporates the E-R model for data modeling.

What happens in a 1:1 relationship?

A single entity instance of one type is related to a single entity instance of another type. Ex.) LOCKER-ASSIGNMENT (only one person can be assigned to one locker)

What happens in a 1:N relationship?

A single entity instance relates to many other entity instances. Ex.) ITEM-QUOTE (one item instance can be related to many quotation instances, but the quotation instances only have one item instance)

What are Inclusive Subtypes?

A supertype instance that can relate to one or more subtypes.

What are Exclusive Subtypes?

A supertype instance that is related to at most one subtype.

What is the special type of weak entity, known as an ID-dependent Entity?

A type of entity in which the identifier of the entity includes the identifier of another entity. For example the entity APARTMENT cannot exist without the entity BUILDING, because logically you must have a building in order for an apartment to exist.

What is Integrated Definition 1, Extended (IDEF1X)?

A version of the E-R model that incorporates the basic ideas of the E-R model but uses different graphical symbols that, unfortunately, make it difficult to understand and use.

What is Information Engineering (IE)?

An E-R Model developed by James Martin, that uses "crows feet" to show the many sides of a relationship, and it is sometimes called the IE Crow's Feet Model.

What is the difference in an Entity Class and an Entity Instance?

An Entity Instance is one individual Entity within each Entity Class. There are many Instances within each Class.

What are Relationship Classes?

An association between entity classes.

What is a Relationship?

An association between two entities, objects, or rows of relations.

What is a Discriminator?

An attribute that determines which subtype is appropriate.

What is a Subtype Entity?

An entity or object that is a subspecies or subcategory of a higher-level type, called a supertype. Ex.) Students may be listed as Undergraduate or Graduate. STUDENT would be the supertype and UNDERGRADUATE or GRADUATE would be the subtype.

What is a Strong Entity?

An entity that can exist in a database on its own.

What is a Weak Entity?

An entity that cannot exist in a database unless another type of entity also exists in that database.

When developing a Data Model, developers do what?

Analyze the forms, reports, queries, and other system requirements.

What is are Relationship Instances?

Associations among entity instances.

What are Identifiers?

Attributes that name, or identify, entity instances.They can be unique or non-unique. Ex.) The identifier of the entity, ITEM, could be, Item Number

What are relationships of degree two most commonly called?

Binary Relationships

During the process of establishing requirements system, what do developers need to document?

Business rules that constrain actions on database activity. Ex.) Students must declare a major, no adviser may have more than 25 advisees, etc.

How can Minimum Cadinality be shown?

By placing a hash mark ( | )across the relationship line to indicate that the entity must exist in the relationship and to place an oval ( 0 ) across the relationship line to indicate that an entity might or might not be in the relationship.

In addition to the Use Case requirements, what do you need to document?

Characteristics of data items.

What are identifiers that consist of two or more attributes?

Composite Identifiers Ex.) (Area Code, Local Number), (Project Name, Task Name), (First Name, Last Name, Phone Extension)

Database Development Process

Consists of three major stages: requirements analysis, component design, and implementation.

Cardinality

Count

What are Use Cases?

Descriptions of the ways users will employ the features and functions of the new information system.

When you create a database, data requirements must first be what?

Documented in a data model.

When you write application programs, program logic must first be what?

Documented in flowcharts or object diagrams.

What are the most important elements of the E-R Model?

Entities, Attributes, Identifiers and Relationships.

What is an example of a business rule for a manufacturing company?

Every purchased part will have a quotation from at least two suppliers.

Entities of a given type are what?

Grouped into an Entity Class. Ex.) EMPLOYEE entity class is a collection of all EMPLOYEE entities

What is the value of a Non-Unique Identifier?

Identifies a set of instances. Ex.) Employee Name, because there can be more than one person with the same name.

What is the value of a Unique Identifier?

Identifies one and only one entity instance. Ex.) Employee Number

How can you tell if an entity's minimum cardinality is optional or mandatory?

If the hash mark is present, the entity's participation is mandatory. If the oval is present, the entity's participation is optional.

Why would you list the entity with the identifier attribute only or with no attributes at all?

In a large data model, too much detail can make the data model diagrams hard to use or manage.

In Use Cases, what is defined?

Inputs provided to the system and outputs generated by the system.

For each data item in a form, report or query, what needs to be determined?

Its data type, properties and limits on values.

What do the constraints consist of, in design stage?

Limits on data values, referential integrity constraints, and business rules.

What happens in a N:M relationship?

Many entities can relate to many other entities. Ex.) An item can be supplied by many suppliers, and a supplier can supply many items.

What is Maximum Cardinality?

Maximum number of entity instances that can occur on each side of the relationship.

The minimum cardinality from the ID-dependent entity to the parent entity is always, what?

One

What does an identifier consist of?

One or more of the entity's attributes.

What are the three Binary Relationships?

One-to-One (1:1) One-to-many (1:N) Many-to-Many (N:M)

When discussing 1:N relationships what terms are sometimes used?

Parent and Child.

What is an example of limits on data values?

Part numbers are seven-digit numbers start with the number 3.

What are Use Cases good for?

Providing sources of requirements and to validate the data model, design, and implementation.

In an E-R diagram what shapes are used to represent what?

Rectangles = entity classes Diamonds = relationships The maximum cardinality of a relationship is shown inside the diamond, and the minimum cardinality is shown by an oval or hash mark next to the entity. The name of the entity is shown inside the rectangle, and the name of the relationship is show near the diamond.

What is the Requirement Analysis stage also referred to as?

Requirements Stage

What is an Entity?

Something that users want to track. Ex.) CUSTOMER John Doe, PURCHASE 12345, PRODUCT A4200, etc.

What is the Component Design Stage also referred to as?

System Design Stage and the Design Stage

What happens in the Requirements Stage?

System users are interviewed and sample forms, reports, queries, and descriptions of update activities are obtained.

What does the database design consist of?

Tables, relationships, and constraints, consisting of table names, table column names, data types and properties of the columns, and a description of primary and foreign keys.

What are relationships of degree three most commonly called?

Ternary Relationships

What is the most popular way to create a data model?

The Extended Entity-Relationship Model (E-R Model)

What is the database development process a subset of?

The Systems Development Life-Cycle (SDLC) model.

What happens during the Design Stage?

The data model is transformed into a database design.

With an Attribute, what is determined by the requirements?

The data type (character, numeric, date, currency, and the like) and properties.

What happens during the Implementation Stage?

The database is constructed in the DBMS and populated with data; queries, forms, and reports are created; application programs are written; and all of it is tested. Also, users are trained, documentation is written, and the new system is put into use.

What does it mean if the minimum cardinality of an entity is one?

The entity's participation in the relationship is mandatory.

What does it mean if the minimum cardinality of an entity is zero?

The entity's participation in the relationship is optional.

How is an entity displayed with all of its attributes?

The identifier of the attribute is shown at the top of the entity, and a horizontal line is drawn after the identifier.

What is one difference in a HAS-A relationship and a IS-A relationship?

The identifiers. In a HAS-A relationship the can be different, but in a IS-A relationship they must be the same.

What is Minimum Cardinality?

The minimum number of entity instances that must participate in a relationship instance.

What is the Degree of the Relationship?

The number of entity classes in the relationship. Ex1.) SUPPLIER-QUOTATION relationship is of degree two because it involves two entity classes: SUPPLIER and QUOTATION. Ex2.) PARENT relationship is of degree three if it involves three entity classes such as: MOTHER, FATHER, and CHILD.

What is an Entity Instance of an entity class?

The occurrence of a particular entity. Ex.) CUSTOMER 12345.

What are Entities usually the subject of, when developing a Data Model?

The subject of one or more forms or reports, or they are a major section in one or more forms or reports.

Why are subtypes used in a data model?

To avoid inappropriate NULL values.

What is the information, obtained in the Requirements Stage, used for?

To create a Data Model.

What do the properties of an attribute specify?

Whether it is required, has a default value, has value limits or any other type of constraint.

What makes a relationship strong or weak?

Whether or not one can logically exist without the other.

What is the principle difference between an entity and a table?

You can express a relationship between two entities without using foreign keys. And because you are doing logical data modeling, and not physical database design, there is no need to worry about primary and foreign keys, referential integrity constraints, and the like.