The Entity-Relationship Model Chapter 2 1 Objectives

The Entity-Relationship Model Chapter 2 1

Objectives v Overview of database design § v Requirement analysis, conceptual design, logical design, normalization, and physical design The entity-relationship (ER) model § Basic constructs: • • • § Advanced constructs: • • v Entities Relationships Attributes (of entities and relationships) Constraints Weak entities ISA hierarchies Aggregation Conceptual design using the ER model 2

Overview of Database Design Requirements analysis: Find out what users want to do with the database § § § v Conceptual design: Use the output of RA to develop a highlevel description of the data to be stored, along with their constraints. Output of CD usually is an ER-diagram. § § v What data should be stored in the database? What applications should be built on top of it? What operations have critical performance requirements? What are entities and relationships of the domain? What are the integrity constraints (= business rules) that hold? Logical design: Choose a DBMS and map the conceptual schema (ER-diagram) into the data model of the DBMS. Output of this step is the logical schema of the data. 3

Overview of Database Design (Cont’d) v v v Schema refinement: Analyze the logical schema, identify potential problems in it, and fix these by refining the logical schema using known normal forms. Physical design: Consider expected workloads that the database will support to further refine the design in order to meet desired performance criteria. Output here is the physical schema. Application and security design: Consider aspects of the design that go beyond the database itself. Complete design methodologies such as UML are useful here: § § § What are the users and processes involved in the database application? What are the roles of the involved users? What parts of the database are accessible to each role and what must not? 4

Conceptual Database Design v Conceptual design: (ER Model is used at this stage. ) § § § What are the entities and relationships in the enterprise? What information about these entities and relationships should we store in the database? Which integrity constraints (business rules) hold? How to represent a database `schema’ in the ER model ? pictorially (as ER diagrams) ! How to map an ER diagram into a relational schema? 5

ER Model Basics ssn name lot Employees v v v Entity: Real-world object distinguishable from other objects. E. g. , one employee. Attribute: Construct describing an entity. E. g. , the name of an employee. Entity Set: A collection of similar entities. E. g. , all employees. § § § v All entities in an entity set have the same set of attributes. (Exception: ISA hierarchies) Each entity set has a key, i. e. a minimal set of attributes whose values uniquely identify an entity in the entity set. Underlined ! Each attribute has a domain, i. e. a set of possible values Entity set ~ rectangle; attribute ~ oval. 6

ER Model Basics (Cont’d. ) since name ssn lot Employees v v v dname did Works_In budget Departments Relationship: Association among 2 or more entities. E. g. , John works in the Pharmacy department. Relationship Set: Collection of similar relationships. § An n-ary relationship set R relates n entity sets E 1. . . En; each relationship in R involves entities e 1 of E 1, . . . , en of En. Relationship Set ~ diamond. 7

name ER Model Basics (Cont’d. ) ssn lot Employees v v dname did Works_In lot Employees since name ssn budget Departments supervisor subordinate Reports_To Descriptive attributes: Attributes of relationships. A relationship set may relate non-distinct entity sets: § § Related entities play different roles; e. g. , emp 1 reports to a managing employee emp 2. Role indicators emphasize this difference in roles. Unique names for attributes of the relationship set are obtained by concatenating the indicator with the attributes of the entity set. 8

Key Constraints v v Consider Works_In: An employee can work in many departments; a dept can have many employees. In contrast, each dept has at most one manager; this is an example of a key constraint on Manages, also called 1 -to-many relationship set. (See arrow in figure) since name ssn dname lot Employees 1 -to-1 1 -to Many did Manages Many-to-1 budget Departments Many-to-Many 9

Participation Constraints v Does every department have a manager? § If so, this is a participation constraint: the participation of Departments in Manages is said to be total (vs. partial). • (Every did value in Departments table must appear in a row of the Manages table with a non-null ssn value!) • Below: thick (thin) lines mean total (partial) participation. since name ssn did lot Employees dname Manages budget Departments Works_In since 10

Weak Entities v A weak entity can be identified uniquely only by considering the primary key of another (identifying owner) entity. § Owner entity set and weak entity set must participate in a one-tomany relationship set (one owner, many weak entities). § Weak entity set must have total participation in this identifying relationship set. § Example below: employees holding insurance policies for their dependents. The weak entity set and its identifying relationship are indicated by drawing them with dark lines. § A weak entity set has a partial key (indicated with dashed line). name ssn lot Employees cost Policy pname age Dependents 11

name ssn ISA (`is a’) Hierarchies hourly_wages As in C++, or other OO PLs, attributes are inherited. If we declare A ISA B, then every A entity is also a B entity. v v v lot Employees hours_worked ISA contractid Hourly_Emps Contract_Emps Overlap constraints: Are two classes allowed to overlap? E. g. , can Joe be both an Hourly_Emps and a Contract_Emps entity? (Allowed/disallowed) Covering constraints: Do entities of subclasses contain all those in the superclass? E. g. , does every Employees entity also have to be an Hourly_Emps or a Contract_Emps entity? (Yes/no) Reasons for using ISA (by specialization/generalization): § To add descriptive attributes specific to a subclass. § To identify entities that participate in a relationship (See slide 17). 12

ssn Aggregation v § Aggregation: Allows to treat a relationship set as an entity set for purposes of participation in (other) relationships. Notation: dashed box indicating that a relationship set participates in another relationship set. lot Employees Used to model a relationship between entity sets and relationship sets. § name Monitors since started_on pid pbudget Projects until dname did Sponsors budget Departments * Aggregation vs. ternary relationship: -- Monitors is a distinct relationship, with a descriptive attribute. -- Also, can say that each sponsorship is monitored by at most one employee. 13

Conceptual Design Using the ER Model v Design choices: § § § v Should a concept be modeled as an entity or an attribute? Should a concept be modeled as an entity or a relationship? Identifying relationships: Binary or ternary? Aggregation? Constraints in the ER Model: § § A lot of data semantics can (and should) be captured. But some constraints cannot be captured in ER diagrams. 14

Entity vs. Attribute Should address be an attribute of Employees or an entity (connected to Employees by a relationship)? v Depends upon the use we want to make of address information, and the semantics of the data: v • If we have several addresses per employee, address must be an entity (since attributes cannot be setvalued). • If the structure (city, street, etc. ) is important, e. g. , we want to retrieve employees in a given city, address must be modeled as an entity (since attribute values are atomic). 15

Entity vs. Attribute (Contd. ) from name v v v Works_In 4 does not allow an employee to work in a department for two or more periods. Similar to the problem of wanting to record several addresses for an employee: We want to record several values of the descriptive attributes for each instance of this relationship. Accomplish this by introducing new entity set Duration. ssn to lot did Works_In 4 Employees ssn name dname lot Employees from budget Departments did Works_In 4 Duration dname budget Departments to 16

Entity vs. Relationship v v v Should an attribute be descriptive or not? First ER diagram depicts the situation where a manager gets a separate discretionary budget for each dept. What if a manager gets a discretionary budget that covers all managed depts? § § Redundancy: dbudget stored for each dept managed by manager. Misleading: Suggests dbudget associated with department-mgr combination. since name ssn dbudget lot Employees did dname budget Departments Manages 2 name ssn lot since Employees ISA Managers Manages 2 dbudget dname did budget Departments This fixes the problem! 17

Binary vs. Ternary Relationships ssn name pname lot Employees age Dependents Covers Bad design for the additional requirements Policies policyid v cost This ER diagram expresses the following requirements : § An employee can own several policies. § Each policy can be owned by several employees. § Each dependent can be covered by several policies. v The ER diagram doesn’t express the following requirements: § A policy cannot be covered jointly by 2 or more employees. § Every policy must be covered by some employee. § Dependents is a weak entity set with partial key “pname” and identified by the primary key of Policies. v Which constraints would capture the additional requirements? 18

Binary vs. Ternary Relationships (Cont’d) v v Solution: instead of Covers, introduce two binary relationships, Purchaser and Beneficiary. Then add the following constraints: § Key constraint on Policies with respect to Purchaser § Total participation constraint on Policies with respect to Purchaser § Appropriate constraints on the weak entity set Dependents v What if we only add a key constraint on Policies wrt to Covers? ssn name pname lot age Dependents Employees Purchaser Beneficiary Better design policyid Policies cost 19

Binary vs. Ternary Relationships (Contd. ) Previous example illustrated a case when two binary relationships were better than one ternary relationship. v An example in the other direction: a ternary relation Contracts relates entity sets Parts, Departments and Suppliers, and has descriptive attribute qty. No combination of binary relationships is an adequate substitute: v § § S “can-supply” P, D “needs” P, and D “deals-with” S does not imply that D has agreed to buy P from S. How do we record qty? Impossible to represent it cleanly with these binary relations. 20

Summary of Conceptual Design v Conceptual design follows requirements analysis, § v Yields a high-level description of data to be stored ER model popular for conceptual design § § Constructs are expressive, close to the way people think about their applications. It is a semantic model ! Basic constructs are: entities, relationships, and attributes (of entities and relationships). v Some additional constructs are: weak entities, ISA hierarchies, and aggregation. v There are many variations of ER model in the literature. v 21

Summary of ER (Contd. ) v Several kinds of integrity constraints can be expressed in the ER model: key constraints, participation constraints, and overlap/covering constraints for ISA hierarchies. Some foreign key constraints are also implicit in the definition of a relationship set. § § Some constraints (notably, functional dependencies) cannot be expressed in the ER model. Constraints play an important role in determining the best database design for an enterprise. 22

Summary of ER (Contd. ) v ER design is subjective. There are often many ways to model a given scenario! Analyzing alternatives can be tricky, especially for a large enterprise. Common choices include: § Entity vs. attribute, entity vs. relationship, binary or nary relationship, whether or not to use ISA hierarchies, and whether or not to use aggregation. 23