Data Modeling and the Entity-Relationship Model CS 475 Lecture Notes

CS Spring Outline of Lecture Overview of Database Modeling Entity-Relationship Model –Constructs –Relationships –Constraints Entity-Relationship Diagram Chapter 11, Sec

CS Spring Database Modeling Analysis of what information the database must hold Relationships among components of that information Database schema is specified in one of several languages or notations suitable for expressing designs Next, design is committed to a form which can be input to a DBMS  DB takes on physical existence Design notation in this course is Entity -Relationship (E/R) model Ideas ODL, UML, etc. E/R Object-Oriented DBMS Relational DBMS Relations

CS Spring Entity-Relationship Model Principal building blocks: –Entity: Real-world object, concept, or event –Entity type/set: Collection of entities of the same type (same properties); sets don’t have to be disjoint –Attribute: Values describing some property of an entity simple vs. composite single-valued vs. multi-valued stored vs. derived Null –Relationship: Association between two or more entity sets –Relationship type/set: Collection of relationships of the same type

CS Spring Examples - “Movie Database” Entity: –Star Attributes: –Name : “Harrison Ford” (simple, single-valued, stored) –Address : “123 Main Str., LA, CA” (complex) –Birthdate : “1-1-50” (simple) –Age : 50 (could be derived from Birthdate and current date) Entity Type: –Stars (Name, Address, Birthdate, Age) Entity Occurrence –a uniquely identifiable object of an entity type

CS Spring Entity Type in ERD STAR Represented by a rectangle with the entity name in it

CS Spring Attributes Attribute domain - set of allowable values Simple or Composite surname simple address composite (street,city,state) Single valued / multi-valued course_number contact_info - (phone, ,cell,etc) derived - may be generated from a set of attribute values, possibly from different entities.

CS Spring Relationships Define associations among entities Grouped together into relationships sets (types) Example: –Relationship type: Stars-in between Movies and Stars Can be visualized as a table Each entry is a relationship occurrence MoviesStars A. SchwarzeneggerTotal Recall Sharon StoneTotal Recall Sharon StoneBasic Instinct Relationships Relationship Type

CS Spring ERD Relationship StarMovie Stars in Represented by a diamond shape containing the relationship name

CS Spring Relationships Cont’d Degree of a relationship type is the number of participating entity types –binary, ternary, n-ary Example: –Entities: Stars, Movies, Studios –Ternary Relationship Set: Contracts Each entity type that participates in a relationship type plays a role in the relationship –Marked by (optional) name –Entity type may play multiple roles in a relationship type –Ex.: Relationship type Sequel-of between Movies and itself ( Movies )

CS Spring Relationships Cont’d Relationships can have attributes Example: Relationship type Contracts between Stars and Studios for Movies Might want to record salary associated with each contract Not necessary to place attributes on relationships; instead, can invent new entity type whose entities have the attributes ascribed to relationship

CS Spring Constraints on Relationships Limit the possible combinations of entities that participate in the relationship type –Derived from real-world scenario which is being modeled Multiplicity (Cardinality ratio) and participation constraints Cardinality ratio specifies the number of relationship occurrences an entity can participate in –1:1, 1:N, N:1, N:M

CS Spring Mapping Cardinalities Studio-Owns between Studio and Movies is 1:N Stars-in between Stars and Movies is M:N Teaches between Professors and Students is M:N Manages between staff and Branch is 1:1

CS Spring Relationship Constraints Cont’d Existence Dependency Existence of some entity x from entity type X depends on existence of some entity y from entity type Y –x is said to be existence dependent on y –if y is deleted, so is x Example: –Entity type Loans and Payments with relationship Loan-payment (1:N from Loans to Payments ) –Payment entities are existence dependent on loan entities

CS Spring Strong and Weak Entities Strong Entity type An entity type that is not existence-dependent on some other entity type. Weak Entity type an entity type that is existence-dependent on some other entity type – two rectangles

CS Spring Relationship Constraints Cont’d Participation constraint Participation of an entity type E in relationship type R: total or partial –Mandatory (Total) if every entity in E participates in at least one relationship in R –Optional (Partial) if only some entities in E participate in R –Represented by the min value in a min-max pair In previous example, participation of Payment entities in the relationship type Loan-payments is total

CS Spring ER Diagrams - Notation Entity Set Relationship Set Attribute Multivalued Derived... Composite Attribute E1E1 E2E2 R Total participation of E 2 in R E1E1 E2E2 R N1 1:N Relationship between E 1 and E 2 in R Role Name 1:n