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)