BITM 330 chapter 4

discriminator

an attribute of the supertype may be included that indicates which of the subtypes is appropriate for a given instance

a weak entity

an entity that cannot exist in the database without the existence of another entity

strong entity

any entity that is not a weak entity

Associative entity (AKA association entity)

is used when there are attributes that are associated with the relationship between two entities rather than with either of the two entities themselves.

Recursive Relationships

it is possible for an entity to have a relationship to itself

a new entity is then created to

link the two original entities hold the attributes

Many-to-many binary relationship

many entity instances in one entity class is related to many entity instances in another entity class example: a supplier may supply several items and a particular item may be supplied by several suppliers

Business rules

may constrain the model and need to be recorded.

Non-ID-Dependent (weak entity identifier)

may have a single or composite identifier, but the identifier of the parent entity will be a foreign key.

The three stages of database development are

requirements analysis stage, componenet design stage, implementation stage

identifying relationship

the relationship between a strong and weak entity is termed this if the the weak entity is ID-dependent represented by a solid line

nonidentifying relationship

the relationship between a strong and weak entity is termed this if the weak entity is non-ID-dependent represented by a dashed line also used between strong entities

IS-A relationships

the relationships that connect supertypes and subtypes because a subtype is the same entity as the supertype.

inclusive (subtype)

the supertype can relate to one or more subtypes

exclusive (subtype)

the supertype relates to at most one subtype

HAS-A relationships

the term is used because each entity instance has a relationship to a second entity instance. an employee has a badge and a badge has an employee

sources of requirements

user interviews, forms, reports, queries, use cases, business rules

Information Engineering (entity-relationship diagrams)

uses "crow's feet" to show the many sides of a relationship, and it is sometimes called the CROW'S FEET MODEL

ID-Dependent Weak Entities

weak entity that cannot exist without its parent entity. has a composite identifier. the first part of the identifier is the identifier for the strong entity. the second part of the identifier is the identifier for the weak entity itself

4 Problems with Relational Database Model

(multiple data entries, multiple groups of data, time depending data) SOLVABLE (Data Subtypes) NEEDS ADDITIONAL PROGRAMMING

After the requirements have been gathered they are transformed into an...

Entity Relationship (E-R) Data Model

Composite (identifier types)

a composite identifier consists of two or more attributes

Unified modeling language (UML)

a set of structures and techniques for modeling and designing object-oriented programs (OOP) and applications

one-to-one Binary relationship

a single entity instance in one entity class is related to a single entity instance in another entity class. example: an employee may have no more than one locker; a locker may only be accessible by one employee

one-to-many binary relationship

a single entity instance in one entity class is related to many entity instances in another entity class. example: a quotation is associated with only one item and an item may have several quotations

Cardinality

a word that means count. Relationships are named and classified by this. each of the three types of binary relationships shown above have different maximum cardinalities.

relationships

entities can be associated with one another in relationships. relationship degree defines the number of entity classes participating in the relationship

attributes

entities have these and they describe the entity's characteristics: ProjectName, StartDate, ProjectType, ProjectDescription attributes have a data type and properties

Most important elements of E-R Models are

entities, attributes, identifiers, relationships

entity-relationship diagrams

entity classes are shown by rectangles relationships are shown by diamonds the max cardinality of the relationship is shown inside the diamond the min cardinality is shown by the oval or hash mark next to the entity the name of the entity is shown inside the rectangle the name of the relationship is shown near the diamond

identifiers

entity instances have indentifiers. an identifier will identify a particular instance in the entity class: SocialSecurityNumber, StudentID, EmployeeID

subtype

entity is a special case of another entity called supertype

4 typical Business Rules

good selection of PK-relationships normalization data definitions/constraints triggering

Uniqueness (Identifier Type)

identifiers may be unique or nonunique. if the identifier is unique, the data value for the identifier must be unique for all instances

degree 2

is a binary relationship

entity class

is a description of the structure and format of the occurrences of the entity

entity instance

is a specific occurence of an entity within an entity class

degree 3

is a ternary relationship

Integrated Definition 1, extended 3 (IDEF1X)

is a version of the E-R model that is a national standard

prototyping

is commonly used to validate forms and reports. after the data modelhas been completed, it needs to be validated.

max cardinality

is the maximum number of entity instances that may participate in a relationship instance- one, many, or some other fixed number.

minimum cardinality

is the minimum number of entity instances that must participate in a relationship instance. These values typically assume a value of zero (optional) or one (mandatory)