The Entity-Relationship Model

1. The Entity-Relationship Model Chapter 2

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

3. Overview of Database Design Requirements analysis: Find out what users want to do with the database • What data should be stored in the database? • What applications should be built on top of it? • What operations have critical performance requirements? • Conceptual design: Use the output of RA to develop a high-level description of the data to be stored, along with their constraints. Output of CD usually is an ER-diagram. • 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.

4. Overview of Database Design (Cont’d) • 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?

5. Conceptual Database Design • 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?

6. name ssn lot Employees ER Model Basics • 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. • 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.

7. ER Model Basics (Cont’d.) since name dname ssn budget lot did • 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 E1 ... En; each relationship in R involves entities e1 of E1, ..., en of En. • Relationship Set~ diamond. Works_In Employees Departments

8. name ER Model Basics (Cont’d.) ssn lot Employees since name dname • Descriptive attributes: Attributes of relationships. • A relationship set may relate non-distinct entity sets: • Related entities play differentroles; e.g., emp1 reports to a managing employee emp2. 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. super-visor subor-dinate ssn budget lot did Reports_To Works_In Employees Departments

9. since name dname ssn lot Employees Manages Key Constraints did budget • 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 constrainton Manages, also called 1-to-many relationship set. (See arrow in figure) Departments 1-to-1 1-to Many Many-to-1 Many-to-Many

10. Participation Constraints • 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 since name name dname dname ssn did did budget budget lot Departments Employees Manages Works_In since

11. Weak Entities • A weak entity can be identified uniquely only by considering the primary key of another (identifyingowner) entity. • Owner entity set and weak entity set must participate in a one-to-many 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 cost pname age ssn lot Policy Dependents Employees

12. name ssn lot ISA (`is a’) Hierarchies Employees hours_worked 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. hourly_wages ISA • 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 attributesspecific to a subclass. • To identify entities that participate in a relationship (See slide 17). contractid Contract_Emps Hourly_Emps

13. Employees name ssn lot Aggregation Monitors until • Used to model a relationship between entity sets and relationship sets. • 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. since started_on dname pid pbudget did budget Sponsors Departments Projects • 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.

14. Conceptual Design Using the ER Model • Design choices: • 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.

15. Entity vs. Attribute • Should addressbe an attribute of Employees or an entity (connected to Employees by a relationship)? • Depends upon the use we want to make of address information, and the semantics of the data: • If we have several addresses per employee, address must be an entity (since attributes cannot be set-valued). • 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).

16. name dname ssn lot did Employees dname did budget Duration to from Entity vs. Attribute (Contd.) to from budget • Works_In4 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. Departments Works_In4 name ssn lot Works_In4 Departments Employees

17. Entity vs. Relationship since dbudget name dname • 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. ssn lot did budget Departments Employees Manages2 name ssn lot dname since did Employees budget Departments Manages2 ISA This fixes the problem! Managers dbudget

18. name ssn lot Employees Policies policyid cost Binary vs. Ternary Relationships pname age Dependents Covers • 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. • 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. • Which constraints would capture the additional requirements? Bad design for the additional requirements

19. name ssn lot Employees Beneficiary Policies policyid cost Binary vs. Ternary Relationships (Cont’d) • 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 • What if we only add a key constraint on Policies wrt to Covers? pname age Dependents Purchaser Better design

20. Binary vs. Ternary Relationships (Contd.) • Previous example illustrated a case when two binary relationships were better than one ternary relationship. • 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: • 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.

21. 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, 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). • Some additional constructs are: weak entities, ISA hierarchies, and aggregation. • There are many variations of ER model in the literature.

22. Summary of ER (Contd.) • Several kinds of integrity constraints can be expressed in the ER model: key constraints, participationconstraints, 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.

23. Summary of ER (Contd.) • 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 n-ary relationship, whether or not to use ISA hierarchies, and whether or not to use aggregation.