The Entity-Relationship Diagram for Data Modeling

Overview of Database Design 1 Steps in building a database for an application Requirement analysis. Specify what need to be modeled in a real-world domain Conceptual design. Specify a high level schema using a database design model Logical design. Refine a conceptual design to a schema using the data model of a DBMS Physical design. Define the schema using the DDL of the DBMS Database creation. Load data into a database

Overview of Database Design 2 Real-world domain Database design model (Conceptual) (ERD, ODL) DBMS data model (Logical) (relational, OO, OR) Design-to-DBMS step may be automated (e.g., ER-to-relational) Middle step sometimes skipped (directly to relational model)

Requirement Analysis Need to interact with domain experts Interviews and/or Q&A Application specific Time consuming Very important Difficult to be thorough

A Sample Requirement Consider a regional hospital. There are doctors, patients, nurses, prescriptions, tests, … Each doctor has ID, Name, Specialty, Age, Sex, … Patients see doctors, doctors prescribe drugs to patients, … Each patient has one primary care physician, a nurse is assigned to at most 5 patients, …

Conceptual Design Design a high level schema based on user requirements Why do we do a conceptual design? To communicate with domain experts (use concepts of the application domain). It is easier to set up a good model (focus on data content not structure) Flexible (system independent) How to describe a conceptual design? Use an ER Diagram (most popular), ODL

Entity-Relationship Model Proposed by Peter Chen in 1976 Historically very popular Graphical Major concepts: entity type, relationship type, attribute, constraints, …

Entity-Relationship Diagrams 2017-04-28 Entity-Relationship Diagrams Three principal components Entity set (= class), Entity (= object) Attributes Relationship has one name in both directions can involve more than two entity sets

Entity Entity Entity Type A real-world object distinguishable from other objects. Each entity is described by a set of attributes (or properties). Realized as Tuple or Record cf) Object Entity Type A definition of a collection of entities with identical set of attributes. cf) Class Students ID name school

Relationship Relationship Relationship Type An association between two or more entities (usually of different types). A relationship may also have attributes. Relationship Type A definition of a collection of relationships with the identical set of attributes. Address Name Qty PID Order Product Customer ID Date

Attributes Domain Types of attributes The set of values the attribute may take. Types of attributes simple vs composite, single-value vs multi-value, stored vs derived Students ID gpa name fname minit lname hobby bdate age

Degree of Relationships Number of entity types involved in a relationship type. unary, binary, ternary, … Role Part played by entities of a unary relationship. Reports_To lot name subor-dinate super-visor ssn Employees name city S# Suppliers P# color Parts Order Projects J# dept qty

Key Constraint Key Attribute(s) Key Constraint A set of attributes of an entity type whose values are unique for each entity of that type. Key Constraint Every (almost!) entity type must have a key attribute(s). (e.g., ID of Students)

Participation Constraint Whether or not every entity in an entity type participates in a type of relationship Either total or partial gpa grade name takes Students ID title credit cno Courses Every course is taken by some students

Cardinality Constraints Cardinality Ratio. Whether or not an entity of a type may involve in a type of relationship with multiple entities of another entity type. One-to-one (1-to-1). Each entity in E1 is associated with 0 or one entity in E2, and vice versa. Employees manages Departments ssn dno name age since room 1 cf) Other notation exists (James Martin’s IE, IDEF1X,..)

Cardinality Constraints One-to-many (1-to-m). Each entity in E1 is associated with 0 or more entities in E2, and each entity in E2 is associated with 0 or one entity in E1. The reverse is many-to-one (m-to-1). Professors advises Students ssn ID name room gpa 1 m

Cardinality Constraints Many-to-many (m-to-n). Each entity in E1 is associated with 0 or more entities in E2, and vice versa This is the default case if not labeled ID Students takes Courses cno name gps title credit m n grade

Cardinality Every F participate to Relationship R 1 1 R E F 1 m R E F m n R E F 1 m R E F Every F participate to Relationship R R E F Each E is related to unique F

IE Notation for Cardinality James Martin’s Information Engineering 0 or 1 A B A B exactly 1 A B 0 or more than 1 A B 1 or more than 1 A B more than 1

Multiway Relationships 2017-04-28 Multiway Relationships Relationships involving more than two entity sets Ternary (three-way) or higher-degree relationships are rare. Stars Contracts Movies m n Studios 1 A studio has contracted with a particular star to act in a particular movie. : (studio, star, movie) For a particular star and a movie, there is only one studio. Studio 는 여러 star 와 한 movie 에 출연하는 것에 계약가능 한 star 는 한 studio 와 여러 movie 에 출연하는 것에 계약. Star - Studio, Movie- Studio, Star-Movie 간의 relationship 이 따로 존재하는 경우와는 어떻게 틀리는가?

Roles in Relationships 2017-04-28 Roles in Relationships One entity type can appear two or more times in a single relationship. Label for each edge: “role” Sequel-of Movies Original 1 m Sequel 한 영화의 속편은 여러 개 일 수 있다. 어떤 속편의 original 은 한 개 뿐이다.

Attributes on Relationships 2017-04-28 Attributes on Relationships Stars Contracts Movies m n Studios 1 Salary Stars Contracts Movies m n Studios 1 Salary Salaries Salary 를 추가하고 싶은데 이것을 star 의 attribute 로 하면 : star 가 다른 영화마다 다른 salary 를 받으므로 적당치 않음. Movie 의 attribute 도: movie 에 나오는 star 마다 salary 가 다르므로 적당치 않음. Studio 의 attribute ?? 그러므로 salary 는 (star,movie,studio) 의 attribute 가 되어야 한다. Relationship의 attribute 는 두 번째 그림과 같이 새로운 entity 를 추가함으로써 없애 버릴 수 있다.

Multiway to Binary Relationship 2017-04-28 Multiway to Binary Relationship Multiway relationship -> connecting entity Stars Contracts Movies m Studios Studio of Star Producing Studio 1 Star-of Movies m Contracts 1 Studios Stars Movie-of Producing Studio Studio of Star E/R 을 ODL로 바꿀 때 유용 하다. 과정 Relationship 에 대응하는 connecting entity 를 만들고, Connecting entity로 부터 원래 entity 들로 many-one relationship을 각각 만든다. Role 이 붙어 있는 relationship edge 는 각기 다른 relationship 으로 만든다. Connecting entity 는 tuple 을 나타내는 entity 이다. 그러므로 connecting entity 로 부터 각 original entity 로 가는 relationship 들은 many-one 이다.

Subclass in E/R Diagrams (isa Relationship) 2017-04-28 Subclass in E/R Diagrams (isa Relationship) Movies Cartoons Murder- Mysteries isa voices length title year filmType weapon Stars address name 1 1 1 1

Inheritance in the E/R Model 2017-04-28 Inheritance in the E/R Model In E/R model we do not need a new entity set for a derived entity. Cartoon-MurderMystery is not needed cf) In Object-oriented systems, fourth class Cartoon-MurderMystery is needed Movies, Cartoons, Murder-Myesteries are connected by “isa” relationship. They implies a new fourth entity set together (Cartoon-MurderMystery) ex) in the E-R, the effect of this fourth subclass is obtained by putting components of the movie “Roger-Rabbit” in both the Cartoons and Murder-Mysteries entity type => Join ! ODL 에서는 object 가 반드시 한 class 의 member 이어야 한다. 따라서 Cartoon-MurderMystery 를 반드시 만들어야 한다. E/R 에서는 한 entity 가 isa-hierarchy 로 연결된 몇 개의 entity set 들에 속하는 component 를 가진다고 가정한다. 3개의 entity set Movies, Cartoons, Murder-Mysteris 가 isa relationship 에 의해 연결되어 있으므로, 어떤 component (object) 가 이 세개의 entity set 에 동시에 속하는 것이 가능해 진다. Roger Rabbit 은 Movie 의 모든 attribute 와 relationship + Murder Mystery 의 weapon attribute + Cartonns 의 voice 를 모두 attribute 로 가질 수 있다. 그러나 만일 Roger rabbit 이 이 세개의 entity 이외에 다른 attribute 를 가진다면 다시 새로운 entity “Cartoon-MurderMystery” 를 만들어야 할 것이다.

The Modeling of Constraints 2017-04-28 The Modeling of Constraints Keys: attribute(s) that uniquely identify an object within its class. Single-value constraints: the value in a certain role be unique, ex) keys. Referential integrity constraints: a value should exist in DB. No dangling pointers. Domain constraints: the values should lie within a specific range. General constraints: arbitrary assertion, ex) no more than ten stars be listed for one movie.

2017-04-28 Keys In E/R model: Key for an entity set is a set K of one or more attributes that distinguish an entity E1 from another entity E2. Movie: (title) or (title, year) Studio: (name) Star: (name) or (name, address) Creating an attribute whose purpose is to serve as a key Corporation DB: employee ID US country: social security number

Representing Keys in E/R Model 2017-04-28 Representing Keys in E/R Model Underline at key attributes More than one keys: primary and secondary keys, underline only at primary keys, side remarks for indicating secondary keys title length year filmType Movies

Single Value Constraints in E/R 2017-04-28 Single Value Constraints in E/R By implication, each attribute has a single value. We generally assume that it is possible for a value to be null. If not, side remark. “1” for many-one, one-one relationships: single-value constraints.

Referential Integrity 2017-04-28 Referential Integrity Asserts exactly one value exists in a given role. Ways to enforce referential integrity Forbidding the deletion of referenced object If referenced object is deleted, all referencing objects are deleted as well. When creation time ex) movie creation -> automatic reference to a studio. 예를 들어 movie 의 owner 인 studio 가 delete 되면 그 studio 가 own 하고 있는 모든 movie 의 data 가 delete 되는 것..

Referential Integrity in E/R 2017-04-28 Referential Integrity in E/R 1 Owns Runs Movies Studios Presidents A movie must have its owner studio. A president must have his/her studio. A studio can be alive without president. That Studio entity is required to exist. ( rounded arrowhead ) If a studio is deleted, the president of the studio should be deleted from Presidents DB. rounded arrow 사용

Other Kinds of Constraints 2017-04-28 Other Kinds of Constraints Domain constraints X is integer. X is positive integer. X is between 60 and 240: ODL not support but SQL General constraints Movies Stars-in Stars

Weak Entity Type : First Case 2017-04-28 Weak Entity Type : First Case An entity type’s key is composed of attributes some or all of which belong to another entity set. When an entity is a sub-entity of another entity Crews of Movie Studio Each crew has his number (1, 2, 3, …) But, number is not unique (Other crews in other studio) Key of Crew = Crew’s number + Studio’s name Number name addr Weak entity set 이 생기는 첫번째 이유. 어떤 eneity set E 가 다른 entity set F 의 subunit 일 때 이런 현상이 생긴다. Work 1 Crews Studios

Weak Entity Type : Second Case 2017-04-28 Weak Entity Type : Second Case Connecting entity set (to be introduced to eliminate a multiway relationship) Usually, the connecting entity set have no attribute of its own Its key is formed from the attributes that are the key attributes for the entity sets it connect Salary 두번째 이유

Requirements for Weak Entity Types 2017-04-28 Requirements for Weak Entity Types Weak entity type W should have relationship R with other entity types F offering key attributes K R must be a binary, many-one relationship from W to F. (many-one includes one-one) K should be a key attributes of F If F is itself weak, key K may be from another entity set connected to F by a many-one relationship. If there are several many-one relationship from W to F, then each relationship may be used to supply a copy of the key attributes of F to help form the key of W. 일단 E 의 어떤 attribute 도 E 의 entity 를 unique 하게 define 하지 못하는 경우, E에서 many-one 으로 relate 된 F 로 가서 그 key 를 합쳐서 key로 나타낸다.