Lecture 9: Conceptual Database Design January 27 th, 2003

Building an Application with a DBMS Requirements modeling (conceptual, pictures) –Decide what entities should be part of the application and how they should be linked. Schema design and implementation –Decide on a set of tables, attributes. –Define the tables in the database system. –Populate database (insert tuples). Write application programs using the DBMS –way easier now that the data management is taken care of.

Database Design Why do we need it? – Agree on structure of the database before deciding on a particular implementation. Consider issues such as: –What entities to model –How entities are related –What constraints exist in the domain –How to achieve good designs

Database Design Formalisms 1. Object Definition Language (ODL): – Closer in spirit to object-oriented models – I don’t teach it anymore. 2. Entity/Relationship model (E/R): –More relational in nature. Both can be translated (semi-automatically) to relational schemas ODL to OO-schema: direct transformation (C++ or Smalltalk based system).

2. Entity / Relationship Diagrams Entities Attributes Relationships between entities Product address buys

Keys in E/R Diagrams Every entity set must have a key Product namecategory price

address namessn Person buys makes employs Company Product namecategory stockprice name price

What is a Relation ? A mathematical definition: –if A, B are sets, then a relation R is a subset of A x B A={1,2,3}, B={a,b,c,d}, R = {(1,a), (1,c), (3,b)} - makes is a subset of Product x Company: a b c d A= B= makes Company Product

Multiplicity of E/R Relations one-one: many-one many-many abcdabcd abcdabcd abcdabcd

address namessn Person buys makes employs Company Product namecategory stockprice name price What does this say ?

Multi-way Relationships How do we model a purchase relationship between buyers, products and stores? Purchase Product Person Store Can still model as a mathematical set (how ?)

Q: what does the arrow mean ? A: if I know the store, person, invoice, I know the movie too Rental VideoStore Person Movie Invoice Arrows in Multiway Relationships

Q: what do these arrow mean ? A: store, person, invoice determines movie and store, invoice, movie determines person Rental VideoStore Person Movie Invoice Arrows in Multiway Relationships

Q: how do I say: “invoice determines store” ? A: no good way; best approximation: Q: Why is this incomplete ? Rental VideoStore Person Movie Invoice Arrows in Multiway Relationships

Roles in Relationships Purchase What if we need an entity set twice in one relationship? Product Person Store salesperson buyer

Attributes on Relationships Purchase Product Person Store date

Converting Multi-way Relationships to Binary Purchase Person Store Product StoreOf ProductOf BuyerOf date

From E/R Diagrams to Relational Schema Entity set  relation Relationship  relation

Entity Set to Relation Product namecategory price Product(name, category, price) name category price gizmo gadgets $19.99

Relationships to Relations makes Company Product namecategory Stock price name Makes(product-name, product-category, company-name, year) Product-name Product-Category Company-name Starting-year gizmo gadgets gizmoWorks 1963 Start Year price (watch out for attribute name conflicts)

Relationships to Relations makes Company Product namecategory Stock price name No need for Makes. Modify Product: name category price StartYear companyName gizmo gadgets gizmoWorks Start Year price

Multi-way Relationships to Relations Purchase Product Person Store name price ssnname address Purchase(,, )

3. Design Principles Purchase Product Person What’s wrong? President PersonCountry Moral: be faithful!

Design Principles: What’s Wrong? Purchase Product Store date personName personAddr Moral: pick the right kind of entities.

Design Principles: What’s Wrong? Purchase Product Person Store date Dates Moral: don’t complicate life more than it already is.

Modeling Subclasses The world is inherently hierarchical. Some entities are special cases of others We need a notion of subclass. This is supported naturally in object-oriented formalisms. Products Software products Educational products

Product namecategory price isa Educational ProductSoftware Product Age Groupplatforms Subclasses in E/R Diagrams

Understanding Subclasses Think in terms of records: –Product –SoftwareProduct –EducationalProduct field1 field2 field1 field2 field1 field2 field3 field4 field5

Subclasses to Relations Product namecategory price isa Educational ProductSoftware Product Age Groupplatforms NamePriceCategory Gizmo99gadget Camera49photo Toy39gadget Nameplatforms Gizmounix NameAge Group Gizmotodler Toyretired Product Sw.Product Ed.Product

Modeling UnionTypes With Subclasses FurniturePiece Person Company Say: each piece of furniture is owned either by a person, or by a company

Modeling Union Types with Subclasses Say: each piece of furniture is owned either by a person, or by a company Solution 1. Acceptable, imperfect (What’s wrong ?) FurniturePiecePerson Company ownedByPerson

Modeling Union Types with Subclasses Solution 2: better, more laborious isa FurniturePiece Person Company ownedBy Owner isa

Constraints in E/R Diagrams Finding constraints is part of the modeling process. Commonly used constraints: Keys: social security number uniquely identifies a person. Single-value constraints: a person can have only one father. Referential integrity constraints: if you work for a company, it must exist in the database. Other constraints: peoples’ ages are between 0 and 150.

Keys in E/R Diagrams address namessn Person Product namecategory price No formal way to specify multiple keys in E/R diagrams Underline:

Single Value Constraints makes v. s.

Referential Integrity Constraints CompanyProduct makes CompanyProduct makes

Other Constraints CompanyProduct makes <100 What does this mean ?

Weak Entity Sets Entity sets are weak when their key comes from other classes to which they are related. UniversityTeam affiliation numbersportname

Handling Weak Entity Sets UniversityTeam affiliation numbersportname Convert to a relational schema (in class)