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 system’s 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 –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.

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.

Cardinality: 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  :.

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.