1. CSC 411/511: DBMS Design 1 1 Dr. Nan WangCSC411_L2_ER Model 1 The Entity-Relationship Model (Chapter 2)

2. CSC411_L2_ER ModelDr. Nan Wang 22 Main Phases of Database Design

3. CSC411_L2_ER ModelDr. Nan Wang 3 Overview of 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? – What are the integrity constraints or business rules that hold? – A database `schema’ in the ER Model can be represented pictorially (ER diagrams). – Can map an ER diagram into a relational schema.

4. CSC411_L2_ER ModelDr. Nan Wang 4 ER Model Basics Entity-Relationship (ER) data model –A database can be modeled as a collection of entities and relationship among entities –Widely used to develop an initial database design Entity: Real-world object distinguishable from other objects. –An entity is described (in DB) using a set of attributes. Employees ssn name lot

5. CSC411_L2_ER ModelDr. Nan Wang 55 ER Model Basics (Contd.) Entity Set: A collection of similar entities. E.g., all employees. – All entities in an entity set have the same set of attributes. – Each entity set has a key. Key: a minimal set of attributes whose values uniquely identify an entity in the set – Each attribute has a domain. A domain defines the possible values of each attributes E.g., attribute name might be the set of 20-character string Employees ssn name lot

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7. CSC411_L2_ER ModelDr. Nan Wang 7 ER Model Basics (Contd.) Relationship: Association among two or more entities. –E.g., James works in Pharmacy department. –Descriptive attribute, since –Record the information about the relationship, rather than about one of the participating entities did dname budget Departments lot since name Works_In Employees ssn

8. CSC411_L2_ER ModelDr. Nan Wang 88 ER Model Basics (Contd.) Relationship Set: Collection of similar relationships. Same entity set could participate in different relationship sets lot name dname budgetdid since name dname budgetdid since Manages since Departments Employees ssn Works_In

9. CSC411_L2_ER ModelDr. Nan Wang 99 ER Model Basics (Contd.) Relationship Set: Collection of similar relationships. A relationship might involve two entities in the same entity set –Two entities with different “roles” in same set. subor- dinate super- visor Reports_To lot name Employees ssn

10. CSC411_L2_ER ModelDr. Nan Wang 10 Key Constraints Consider Works_In: An employee can work in many departments; a dept can have many employees. Many-to-Many 1-to-11-to ManyMany-to-1 did dname budget Departments lot since name Works_In Employees ssn

11. CSC411_L2_ER ModelDr. Nan Wang 11 Key Constraints In contrast, each dept has at most one manager, according to the key constraint on Manages. The constraint that each department has at most one manager is an example of a key constraint, and it implies that each Departments entity appear in at most one Manages relationship. Arrow states that given a Department entity, we can uniquely determine the manages relationship in which it appears. Many-to-Many 1-to-11-to ManyMany-to-1 dname budgetdid since lot name ssn ManagesEmployees Departments

12. CSC411_L2_ER ModelDr. Nan Wang 12 Key Constraints Each department has only one manager And each employee can manage only one department. 12 Many-to-Many 1-to-11-to ManyMany-to-1 dname budgetdid since lot name ssn ManagesEmployees Departments

13. CSC411_L2_ER ModelDr. Nan Wang 13 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). If the participation of an entity set in a relationship set is total, the two are connected by a thick line Every Departments entity must appear in an instance of the Manages relationship. lot name dname budgetdid since name dname budgetdid since Manages since Departments Employees ssn Works_In

14. CSC411_L2_ER ModelDr. Nan Wang 14 exercise 14

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19. CSC411_L2_ER ModelDr. Nan Wang 19 Homework Page 53, exercise 2.4 Submit your homework (softcopy, PDF or DOC format) at usm.csc411@gmail.com or hardcopy at class)usm.csc411@gmail.com Due date: Next Thursday (Sept, 5th) 19

20. CSC411_L2_ER ModelDr. Nan Wang 20 Weak Entities Employees can purchase insurance to cover their dependents. Only dependents’ SSN is not needed If an employee quits, any policy owned by the employee is terminated and all relevant policy and dependent info will be deleted from the database. 20 lot name age pname Dependents Employees ssn Policy cost ssn

21. CSC411_L2_ER ModelDr. Nan Wang 21 Weak Entities A weak entity can be identified uniquely only by considering the primary key of another (owner) entity. –(One of two or more candidate keys is designated as the primary key) –(Partial key: the set of attributes of a weak entity set that uniquely identify a weak entity for a given owner 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. lot name age pname Dependents Employees ssn Policy cost

22. CSC411_L2_ER ModelDr. Nan Wang 22 ISA (`is a’) Hierarchies As in C++, or other PLs, attributes are inherited. If we declare A ISA B, every A entity is also considered to be a B entity. Employees is specialized into subclasses Hourly-Emp and Contract_EMps are generalized by Employees name ssn Employees lot Contract_Emps hourly_wages ISA Hourly_Emps contractid hours_worked

23. CSC411_L2_ER ModelDr. Nan Wang 23 ISA (`is a’) Hierarchies (Contd.) Overlap constraints: Can Joe be an Hourly_Emps as well as a Contract_Emps entity? (Allowed/disallowed) Covering constraints: –Does every Employees entity also have to be an Hourly_Emps or a Contract_Emps entity? (Yes/no). Intuitively, No. Reasons for using ISA : – To add descriptive attributes specific to a subclass. – To identify entities that participate in a relationship.

24. CSC411_L2_ER ModelDr. Nan Wang 24 Aggregation Used to model a relationship between a collection of entities and relationships. Aggregation allows us to treat a relationship set as an entity set for purposes of participation in (other) relationships. budget did pid started_on pbudget dname until Departments Projects Sponsors Employees Monitors lot name ssn since

25. CSC411_L2_ER ModelDr. Nan Wang 25 Aggregation (Contd.) Aggregation vs. ternary relationship: why not make Sponsors a ternary relationship? –There are really two distinct relationships (monitors and sponsors), each with attribute of its own. Monitors is a distinct relationship, with a descriptive attribute. –Also, can say that each sponsorship is monitored by at most one employee. –How to set key constraint in aggregation and ternary relationship? budget did pid started_on pbudget dname until Departments Projects Sponsors Employees Monitors lot name ssn since

26. CSC411_L2_ER ModelDr. Nan Wang 26 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.

27. CSC411_L2_ER ModelDr. Nan Wang 27 Entity vs. Attribute Should address be an attribute of Employees or an entity (connected to Employees by a relationship)? Employees ssn name address

28. CSC411_L2_ER ModelDr. Nan Wang 28 Entity vs. Attribute 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). Employees ssn name address

29. CSC411_L2_ER ModelDr. Nan Wang 29 Entity vs. Attribute street citya# name ssn Lives_inEmployees addresses 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).

30. CSC411_L2_ER ModelDr. Nan Wang 30 Entity vs. Attribute (Contd.) name Employees ssn lot Works_In4 from to dname budget did Departments did dname budget Departments lot since name Works_In Employees ssn

31. CSC411_L2_ER ModelDr. Nan Wang 31 Entity vs. Attribute (Contd.) 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. –Accomplished by introducing new entity set, Duration. dname budget did name Departments ssn lot Employees Works_In4 Duration from to name Employees ssn lot Works_In4 from to dname budget did Departments

32. CSC411_L2_ER ModelDr. Nan Wang 32 Entity vs. Relationship First ER diagram OK if a manager gets a separate discretionary budget for each dept. What if a manager gets a discretionary budget (sum) that covers all managed depts? – Redundancy: dbudget stored for each dept managed by manager. – Misleading: Suggests that dbudget associated with department-mgr combination. Manages2 name dname budget did Employees Departments ssn lot dbudget since dname budget did Departments Manages2 Employees name ssn lot since Managersdbudget ISA This fixes the problem!

33. CSC411_L2_ER ModelDr. Nan Wang 33 Binary vs. Ternary Relationships (1) The ER diagram shows –An employee can own several policies (dental, health) –Each policy can be owned by several employees –Each dependent can be covered by several policies age pname Dependents Covers name Employees ssn lot Policies policyid cost lot name age pname Dependents Employees ssn Policy cost

34. CSC411_L2_ER ModelDr. Nan Wang 34 Binary vs. Ternary Relationships (1) If each policy is owned by just 1 employee, and each dependent is tied to the covering policy, this ER diagram is inaccurate. Why? –Key constraint on Policies with respect to Covers implies that a policy can cover only one dependent. age pname Dependents Covers name Employees ssn lot Policies policyid cost

35. CSC411_L2_ER ModelDr. Nan Wang 35 Binary vs. Ternary Relationships (2) Additional constraints in the following ER diagram –A policy cannot be owned jointly by two or more employees (key constraint) –Every policy must be owned by some employees (total participation constraint) –Dependents is a week entity set (uniquely identified by pname and policyid) Beneficiary age pname Dependent s policyid cost Policies Purchase r name Employees ssn lot Better design

36. CSC411_L2_ER ModelDr. Nan Wang 36 Binary vs. Ternary Relationships (3) Previous example illustrated a case when two binary relationships were better than one ternary relationship. An example of ternary relationship –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 (without aggregation) 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?

37. CSC411_L2_ER ModelDr. Nan Wang 37 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. Basic constructs –entities, relationships, and attributes (of entities and relationships). Some additional constructs –weak entities, ISA hierarchies, and aggregation. Note: There are many variations on ER model.

38. CSC411_L2_ER ModelDr. Nan Wang 38 Summary of ER (Contd.) 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.

39. CSC411_L2_ER ModelDr. Nan Wang 39 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. Ensuring good database design –resulting relational schema should be analyzed and refined further. –FD (functional dependency) information and normalization techniques are especially useful.

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43. CSC 411/511: DBMS Design 43 Dr. Nan WangCSC411_L2_ER Model 43 Questions?