Data Modeling Entity Relationship Models Models Used to represent unstructured problems A model is a representation of reality Logical models show what a system is or does. implementation-independent; depict business requirements Physical models show not only what a system is or does, but also how the system is physically and technically implemented.
implementation-dependent depict technical requirements Data modeling is a technique for defining business requirements for a database. technique for organizing and documenting a systems DATA. purpose of data modeling - organize data in a way that is flexible and adaptable to unanticipated business requirements Entity Relationship Diagram (ERD). One method of data modeling Several notations Chen Martin Bachman
Entities STUDENT An entity A concept to abstractly represent all instances of a group of similar things class of persons, places, objects, events, or concepts about which we need to capture and store data. entity instance is a single occurrence of an entity. STUDENT Name . Last Name . First Name . Middle Initial Address
. Street Address . City . State or Province . Country . Postal Code Phone Number . Area Code . Exchange Number . Number Within Exchange Date of Birth Gender Race Major Grade Point Average Attributes and compound attributes Attributes descriptive property or characteristic of an
entity The values for each attribute are defined in terms of three properties: data type, domain, and default data type for an attribute defines what class of data can be stored in that attribute domain of an attribute defines what values an attribute can legitimately take on. default value for an attribute is that value which will be recorded if not specified by the user. Keys Every entity must have an identifier or key An key is an attribute, or a group of attributes, which assumes a unique value for each entity instance A group of attributes that uniquely identifies an instance of an entity is called a concatenated key A primary key is that candidate key which will most commonly be used to uniquely identify a single entity
instance. Any candidate key that is not selected to become the primary key is called an alternate key. Relationships Entities interact with, and impact one another via relationships to support the business mission. relationship is a natural business association that exists between one or more entities. verb phrase describes the relationship. All relationships are implicitly bidirectional, meaning that they can interpreted in both directions. CUSTOMER Customer Number (PK) Customer Name Shipping Address Billing Address Balance Due
ORDER has placed Order Number (PK) Order Date Order Total Cost Customer Number (FK) sold ORDERED PRODUCT INVENTORY PRODUCT Product Number (PK) Product Name Product Unit of Measure Product Unit Price sold as
Ordered Product ID (PK) . Order Number (FK) . Product Number (FK) Quantity Ordered Unit Price at Time of Order Cardinality: C a r d in a lit y I n t e r p r e t a t io n M in im u m In sta n c e s M a x im u m In sta n c e s E x a c t ly o n e 1
1 Z ero o r o ne 0 1 O ne o r m o re 1 m any ( > 1 ) Z ero , o n e, o r m o re 0 m any ( > 1 )
> 1 > 1 M o re th a n o n e G r a p h ic N o t a t io n defines the minimum and maximum number of occurrences of one entity for a single occurrence of the related entity Degree The degree of a relationship is the number of entities that participate in the relationship. A binary relationship has a degree = 2, because two different entities participated in the: relationship .
Relationships may also exist between different instances of the same entity. This is called a recursive relationship COURSE Course Id (Primary Key) . Subject Abbreviation . Course Number Course Title Course Credit has as a prerequisite is a prerequisite for INSTRUCTOR COURSE Instructor ID Code (Primary
Key) Instructor Name . Last Name . First Name . Middle Initial Course ID (Primary Key) . Subject Abbreviation . Course Number Course Title Credit meets as is assigned to SCHEDULED CLASS Scheduled Class ID (Primary Key) . Course ID . Instructor ID
. Room ID Division Number Days of Week Start Time End Time is assigned to ROOM Classroom ID . Building Abbreviation . Room Number Number of Seats Associative Entity entity that inherits its primary key from more than one other entity (parents). Each part of that
concatenated key points to one and only one instance of each of the connecting entities. Generalization: technique wherein the attributes that are common to several types of an entity are grouped into their own entity, called a supertype. entity supertype will have one or more one-to-one relationships to entity subtypes. These relationships are sometimes called IS A relationships Subtype An entity subtype is an entity whose instances inherit some common attributes from an entity supertype, and then add other attributes that are unique to an instances of the subtype.
The subtypes not only inherit the attributes, but also the data types, domains, and defaults of those attributes. In addition to inheriting attributes, subtypes also inherit relationships to other entities. An entity can be both a supertype and subtype. CASE Data models are stored in the repository. In a sense, the data model is metadata that is, data about the business data. Computer-aided systems engineering (CASE) technology, provides the repository for storing the data model and its detailed descriptions.
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