Data Modeling (Entity Relationship Diagram) Farrokh Alemi, Ph.D. Updated by Janusz Wojtusiak (Fall 2009)

Overview of the Course 1.Abstract business process into database requirements 2.Model system requirements into a database, and 3.Use Standard Query Language to gain access to the data.

Previous Lecture Presented Guidelines for Defining Fields in Databases.

Entity An entity is something that has an independent, separate, or self-contained existence Divide the list of fields into entities –Each entity will represent a table –Each field in the table is an attribute of the entity.

Entity NameTypeDef. Attribute 1..n NameDef. Data Type Constr. Logical Data Types String Data Types Binary Data Types Numeric Data Types Date/Time Data Types Entities Describe Data Classes

Shared Features Suggest Entities Examine common features of the fields. –"Patient's first name," "Patient's last name" and "Patient's birthday" suggest an entity called "Patient." –"Type of diagnosis" and "Name of diagnosis" suggest an entity called "Diagnosis." Make sense to others Entity names should imply what it is about

Naming of Entities “Create a name that is unambiguous Use the minimum number of words Do not use physical characteristics of the database Do not use abbreviations Do not use words that restrict the data Do not use words that suggest two concepts Use singular form of the name."

Describing an Entity Always include a brief description of the entity. –Set up expectations about fields The entity "Patient" may be defined as: "The clients in the court diversion program who have mental illness. Information about the patients need to be kept in order to track if their court ordered treatment is working and has been followed."

Rules for Assigning Entities No two entities should be about the same things. –For example, calling one entity "Patient" and another "Client" may create both confusion and inefficiency.

Rules for Assigning Entities Do not assign the same field in two different entities. –Patient's name belongs to the "Patient“ entity and not to “Diagnosis” entity. Satisfy the intuitions of future users of the database

Rules for Assigning Entities Make sure that time based events are separated from time independent fields. –A patient's diagnoses does not belong to the entity "Patients" but to “Visits.”

Rules for assigning entities If several entities share the same fields, see if you can partition the shared fields into a separate entity. –Create a new entity called "Person" and keep shared fields in it. Then if an employee falls ill or is seen in the court, their name does not need to be entered several times once as an employee, next as a patient and last as a court client.

Rules for Assigning Entities Include look-up entities as you proceed. Look up table Main entity

Review & Revise Entities Review the fields within entities for completeness. –Many entities suggest new fields that have not been thought about. Review again to see if the fields fit new entities Share the list with organizational members Documentation –a name, a description, a statement of why it is important to track the entity, and a list of fields that belong to it

Entity Relationships Implied in the very definition of the entity. Three types of relationships: –One to one –One to many –Many to many How one entity is mapped into another is also referred to as cardinality of the entity Cardinality defined

Cardinality Follows Business Rules

Many to Many Relationships Reveal New Entities

Display of Relationships Shows a relationship Shows cardinality

Display of Relationships Shows a relationship Names the relationship

Display of Relationships Different standards IDEF1x Barker’s notation UML ons/modeling/idef1x.htmhttp:// ons/modeling/idef1x.htm ng101.htmlhttp:// ng101.html

Documentation of Relationships Name of both entities The verb phrase that describes the semantics of the linkage The cardinality of the linkage All relationships must be documented before proceeding to the physical design of the database