Instructor: Elke Rundensteiner Database Management Systems CS 542 --- Spring 2017 Modeling Your Data Chapter 2.   Instructor: Elke Rundensteiner

Overview of Database Design What are the entities and relationships in your application? What information store in database? What integrity constraints or business rules hold? 2

ER Model : Database Design A database `schema’ in the ER Model represented pictorially 2

ER Model Basics Entity: Real-world object described by a set of attributes. Entity Set: A collection of similar entities. E.g., all employees. All entities in entity set have same set of attributes. Each entity set has a key. Each attribute has a domain. Employees ssn name lot The slides for this text are organized into several modules. Each lecture contains about enough material for a 1.25 hour class period. (The time estimate is very approximate--it will vary with the instructor, and lectures also differ in length; so use this as a rough guideline.) This covers Lectures 1 and 2 (of 6) in Module (5). Module (1): Introduction (DBMS, Relational Model) Module (2): Storage and File Organizations (Disks, Buffering, Indexes) Module (3): Database Concepts (Relational Queries, DDL/ICs, Views and Security) Module (4): Relational Implementation (Query Evaluation, Optimization) Module (5): Database Design (ER Model, Normalization, Physical Design, Tuning) Module (6): Transaction Processing (Concurrency Control, Recovery) Module (7): Advanced Topics 3

ER Model Basics (Contd.) Relationship: Association among two or more entities. E.g., Attishoo works in Pharmacy department Relationship Set: Collection of similar relationships. An n-ary relationship set R relates n entities e1 from E1, ..., en from En 4

ER Model Basics Relationship: Association among two or more entities. lot dname budget did since name Works_In Departments Employees ssn Relationship: Association among two or more entities. 4

ER Model Basics Relationship: Same entity set can participate in different relationship sets (“roles”) in same set. name ssn lot Employees super-visor subordinate Reports_To 4

Key Constraints Many-to-Many 1-to-1 1-to Many Many-to-1 6

An employee can work in many departments, and since name dname ssn lot did budget Employees Works_In Departments Works_In: An employee can work in many departments, and a dept can have many employees. 4

Which key constraint ? 1-to-1 1-to Many Many-to-1 Many-to-Many since name dname ssn lot did budget Employees Works_In Departments 1-to-1 1-to Many Many-to-1 Many-to-Many 4

Which Key Constraint Case ?? Consider Manager Relation- ship? since lot name ssn dname did budget Manages Employees Departments 6

Which Key Constraint Case ?? since Consider Manager Relation- ship : “Each dept has at most one manager.” lot name ssn dname did budget Manages Employees Departments Many-to-Many 1-to-1 1-to Many Many-to-1 6

Key Constraint: 1 - to - Many since Each dept has at most one manager as per key constraint on Manages. lot name ssn dname did budget Manages Employees Departments dname budget did since lot name ssn Manages Employees Departments [0:1] [0:n] 6

Participation Constraints Must every department have a manager? lot dname did name budget since Manages Departments Employees ssn ? 8

Participation Constraints If every department must have a manager, that is, the participation of Departments in Manages is said to be total (vs. partial). lot name dname budget did since Manages Departments Employees ssn ? 8

Participation Constraints Every department must have a manager (exactly one)! lot name dname budget did since Manages Departments Employees ssn 8

Participation Constraints ? lot dname budget did since name Works_In Departments Employees ssn 8

Participation Constraints lot name dname budget did since Manages Departments Employees ssn Works_In every department has a manager every employee works in at least one dept. every dept has at least one employee 8

Weak Entities Weak entity can be identified uniquely only by considering the primary key of another (owner) entity. lot name age pname Dependents Employees ssn Policy cost 10

Weak Entities Owner entity set and weak entity set must participate in a one-to-many relationship set (one owner and many weak entities). Weak entity set must have total participation in this identifying relationship set. lot name age pname Dependents Employees ssn Policy cost 10

ISA (`is a’) Hierarchies 1. As in C++, or other PLs, attributes are inherited. 2. If we declare A ISA B, every A entity is also considered to be a B entity. Contract_Emps name ssn Employees lot hourly_wages ISA Hourly_Emps contractid hours_worked 12

ISA (`is a’) Hierarchies name Implicit Relationship Between Super- And Subentity? 1-1 ? ssn lot Employees hourly_wages hours_worked ISA contractid Hourly_Emps Contract_Emps Reasons for using ISA: To add descriptive attributes specific to a subclass. To identify entities that participate in a relationship. 12

ISA (`is a’) Hierarchies Contract_Emps name ssn Employees lot hourly_wages ISA Hourly_Emps contractid hours_worked Overlap constraints: Can Joe be Hourly_Emps as well as Contract_Emps entity? (Allowed/disallowed) Covering constraints: Does every Employee entity have to be Hourly_Emps or Contract_Emps entity? (Yes/no) 12

Aggregation Used when we have to model a relationship involving entity sets and a relationship set. 2

Aggregation Aggregation allows us to treat a relationship set as an entity set for purposes of participation in (other) relationships name ssn lot Employees Monitors until started_on since dname pid pbudget did budget Projects Sponsors Departments 2

Aggregation Aggregation vs. ternary relationship: name ssn lot Employees Monitors until started_on since dname pid pbudget did budget Projects Sponsors Departments Aggregation vs. ternary relationship: Monitors is a distinct relationship, with a descriptive attribute. Or, say each sponsorship is monitored by at most one employee. 2

Summary of Conceptual Design Conceptual design follows requirements analysis: Yields a high-level description of data to be stored ER model popular for conceptual design Constructs are expressive - the way people think about their applications. Basic constructs: entities, relationships, and attributes Some additional constructs: weak entities, ISA hierarchies, and aggregation. Note: There are many variations on ER model. 11

Summary of ER Modeling Constraints play important role in determining the best database design for an enterprise. Integrity constraints expressed in ER model: key constraints, participation constraints, overlap/covering constraints for ISA hierarchies. Some foreign key constraints also implicit in definition of a relationship set. Others cannot be expressed : Some constraints (notably, functional dependencies) cannot be expressed in ER model. 12