1. Database Management Systems 1 Raghu Ramakrishnan The Entity-Relationship (ER) Model Chpt 2 Instructor: Jianping Fan http://www.cs.uncc.edu/~jfan

2. Database Management Systems 2 Raghu Ramakrishnan Overview of Database Design a.What is database ? ---database consists of many tables and their inter-table relationships ---each table has some “similar” tuples b. What database should address ? ---tables and their relationships ---submission & processing of queries 3 key issues: Description, Organization & Search

3. Database Management Systems 3 Raghu Ramakrishnan Example: University Database Tables: Students: SID, sname, year, GPA Departments: DID, dname, office Faculties: ssn, fname, f-office, phone, salary Courses: CID, cname, time, room, credit-hour Inter-Table Relationships: Students enroll in Courses, Faculties teach Courses Faculties work for Departments Query on multiple tables is allowed!

4. Database Management Systems 4 Raghu Ramakrishnan Overview of Database Design v Requirement analysis : analysis of data and users’ requirement to entity sets and relationship sets v Conceptual database design : ER model for description v Logical database design : ER model to relational database v Schema refinement : analyze the collections of relations v Physical database design : database indexing v Security design : access control Beyond ER model Requirement ER design Relational DBMS Analysis Schema

5. Database Management Systems 5 Raghu Ramakrishnan Overview of Database Design v Requirement analysis : analysis of data and users’ requirement to entity sets and relationship sets Requirement ER design Relational DBMS Analysis Schema a.What kind of attributes should be included for tuple description? b.Which attributes should be indexed? Frequent operations & access c.Long schema or short schema (balance of storage & access efficiency)? d.Table integration or separation? Updating frequency e.Query optimization framework?

6. Database Management Systems 6 Raghu Ramakrishnan Overview of Database Design v Conceptual design : (ER Model is used at this stage.) a.Should a concept be modeled as an entity or an attribute? b.Should a concept be modeled as an entity or a relationship? c.What are relationship sets and their participating entity sets? d.Should we use binary or ternary relationships? e. Should we use aggregation?

7. Database Management Systems 7 Raghu Ramakrishnan Overview of Database Design v Logical database design : ER model to relational database Requirement ER design Relational DBMS Analysis Schema a.How to create physical database tables? b.How to transform E-R models (conceptual database design) into physical database tables? a.How generate physical database indexing for fast query?

8. Database Management Systems 8 Raghu Ramakrishnan Overview of Database Design v Schema refinement : analyze the collections of relations Requirement ER design Relational DBMS Analysis Schema a.Which tables should be separated into multiple smaller tables? b.Which tables can be integrated as one single larger table? c.Which attributes should be inserted into existing schema? d.Which attributes can be deleted from existing schema?

9. Database Management Systems 9 Raghu Ramakrishnan Overview of Database Design v Physical database design : database indexing Requirement ER design Relational DBMS Analysis Schema a.Which attributes should be selected for indexing? Most frequently-used attributes for query formulation b.What kind of indexing structures should be selected? range search or equal search?

10. Database Management Systems 10 Raghu Ramakrishnan Overview of Database Design v Security design : access control Requirement ER design Relational DBMS Analysis Schema Who can access what in database under which conditions?

11. Database Management Systems 11 Raghu Ramakrishnan Overview of 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 (i.e., attributes )? –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.

12. Database Management Systems 12 Raghu Ramakrishnan University Database v University database contains employees and departments which are described by certain attributes v University database also contains relationships between employees departments which are also described by certain attributes

13. Database Management Systems 13 Raghu Ramakrishnan ER Model Basics v Entity: Real-world object distinguishable from other objects. An entity is described (in DB) using a set of attributes. Employees ssn name lot 999-80-3267, John Smith, 2003, 3.5 999-32-0847, James Gary, 2006, 3.0 ID name year GPA

14. Database Management Systems 14 Raghu Ramakrishnan ER Model Basics v 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. –Each attribute has a domain. Employees ssn name lot What’s the key? How many keys one object can have?

15. Database Management Systems 15 Raghu Ramakrishnan Entity, Entity Set, Attribute, Schema & Domain ID or SSN NameYear Age GPA 999-38-4431John Smith 1999 213.68 999-28-3341 Miki Jordan 2000 28 3.45 331-43-4567 David Kim2000 25 4.00 535-34-5678 Paul Lee 1998 26 3.89

16. Database Management Systems 16 Raghu Ramakrishnan ER Model Basics (Contd.) v Relationship : Association among two or more entities. v Examples: Fan works in Computer Science Department. Smith work in Electronic Engineering Department lot dname budget did since name Works_In Departments Employees ssn Relationship Set

17. Database Management Systems 17 Raghu Ramakrishnan ER Model Basics (Contd.) v 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 E1,..., en En u Same entity set could participate in different relationship sets, or in different “roles” in same set. lot dname budget did since name Works_In Departments Employees ssn Relationship Set

18. Database Management Systems 18 Raghu Ramakrishnan Entity vs. Entity Set Student --- Students John Smith(999-21-3415, jsmith@, John Smith, 18, 3.5) Students in ITCS3160 999-21-3415, jsmith@, John Smith, 18, 3.5 999-31-2356, jzhang@, Jie Zhang, 20, 3.0 999-32-1234, ajain@, Anil Jain, 21, 3.8

19. Database Management Systems 19 Raghu Ramakrishnan Entity Keys 999-21-3415, jsmith@, John Smith, 18, 3.5 999-31-2356, jzhang@, Jie Zhang, 20, 3.0 999-32-1234, ajain@, Anil Jain, 21, 3.8 Primary key Candidate key

20. Database Management Systems 20 Raghu Ramakrishnan Relationship vs. Relationship Set John Smith ITCS3160 (999-21-3415, jsmith@, John Smith, 18, 3.5) (3160, ITCS, DBMS, J. Fan, 3, Kenn. 236) Relationship

21. Database Management Systems 21 Raghu Ramakrishnan Relationship vs. Relationship Set 999-21-3415, jsmith@, John Smith, 18, 3.5 999-31-2356, jzhang@, Jie Zhang, 20, 3.0 999-32-1234, ajain@, Anil Jain, 21, 3.8 3160, ITCS, DBMS, J. Fan, 3, Kenn. 236 6157, ITCS, Visual DB, J. Fan, 3, Kenn. 236 Relationship set

22. Database Management Systems 22 Raghu Ramakrishnan Many-to-Many 1-to-1 1-to ManyMany-to-1 Potential Relationship Types

23. Database Management Systems 23 Raghu Ramakrishnan Example 1 v Build an ER Diagram for the following information: – Students u Have an Id, Name, Login, Age, Gpa – Courses u Have an Id, Name, Credit Hours –Students enroll in courses u Receive a grade

24. Database Management Systems 24 Raghu Ramakrishnan Example 1 Answer Students Name Login Id AgeGPA Courses Id NameCredit Enrolled_In Grade

25. Database Management Systems 25 Raghu Ramakrishnan Example 2 v Build an ER Diagram for the following information: – Patients u Name, Address, Phone #, Age – Drugs u Name, Manufacturer, Expiration Date –Patients are prescribed drugs u Dosage, # Days

26. Database Management Systems 26 Raghu Ramakrishnan Example 2 Answer Patients Name AddrPhoneAge Drug Name ManufExp Prescribed Dosage #days

27. Database Management Systems 27 Raghu Ramakrishnan Example 3 v Build an ER Diagram for the following information: – Students u Have an Id, Name, Login, Age, Gpa – Courses u Have an Id, Name, Credit Hours –Students enroll in courses u Receive a grade - faculties u Name, Address, Phone #, Age – Faculties teach courses u semester

28. Database Management Systems 28 Raghu Ramakrishnan Entity vs. Attribute: Ternary Relationship v 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: u If we have several addresses per employee, address must be an entity (since attributes cannot be set-valued). u 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).

29. Database Management Systems 29 Raghu Ramakrishnan v Works_In2 does not allow an employee to work in a department for two or more periods. v 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. name Employees ssn lot Works_In2 from to dname budget did Departments dname budget did name Departments ssn lot Employees Works_In3 Duration from to Same employee works in same department in different periods

30. Database Management Systems 30 Raghu Ramakrishnan Entity vs. Attribute v 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: u If we have several addresses per employee, address must be an entity (since attributes cannot be set-valued). u 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).

31. Database Management Systems 31 Raghu Ramakrishnan v Works_In2 does not allow an employee to work in a department for two or more periods. v 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. name Employees ssn lot Works_In2 from to dname budget did Departments dname budget did name Departments ssn lot Employees Works_In3 Duration from to Entity vs. Relationship: Ternary Relationship

32. Database Management Systems 32 Raghu Ramakrishnan Entity vs. Relationship: Ternary Relationship v First ER diagram OK if a manager gets a separate discretionary budget for each dept. –Redundancy of dbudget, which is stored for each dept managed by the manager. –Misleading: suggests dbudget tied to managed dept. v What if a manager gets a discretionary budget that covers all managed depts?

33. Database Management Systems 33 Raghu Ramakrishnan Entity vs. Relationship: Ternary Relationship v First ER diagram OK if a manager gets a separate discretionary budget for each dept. –Redundancy of dbudget, which is stored for each dept managed by the manager. –Misleading: suggests dbudget tied to managed dept. v What if a manager gets a discretionary budget that covers all managed depts? Manages2 name dname budget did Employees Departments ssn lot dbudget since Employees since name dname budget did Departments ssn lot Mgr_Appts Manages3 dbudget apptnum

34. Database Management Systems 34 Raghu Ramakrishnan Binary vs. Ternary Relationships v If each policy is owned by just 1 employee: –Key constraint on Policies would mean policy can only cover 1 dependent! age pname Dependents Covers name Employees ssn lot Policies policyid cost Beneficiary age pname Dependents policyid cost Policies Purchaser name Employees ssn lot Bad design Better design *

35. Database Management Systems 35 Raghu Ramakrishnan dname budget did name Departments ssn lot Employees Works_In3 Duration from to Binary vs. Ternary Relationships (Contd.) dname budget did name Departments ssn lot Employees Works_In3 from to

36. Database Management Systems 36 Raghu Ramakrishnan Binary vs. Ternary Relationships (Contd.) v 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 set Parts, Departments and Suppliers, and has descriptive attributes 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?

37. Database Management Systems 37 Raghu Ramakrishnan lot dname budget did since name Works_In Departments Employees ssn Key Constraints v Consider Works_In: An employee can work in many departments; a dept can have many employees.

38. Database Management Systems 38 Raghu Ramakrishnan lot dname budget did since name Works_In Departments Employees ssn Key Constraints v Consider Works_In: An employee can work in at most one department; a dept can have many employees.

39. Database Management Systems 39 Raghu Ramakrishnan Key Constraints v In contrast, each dept has at most one manager, according to the key constraint on Manages. dname budgetdid since lot name ssn Manages Employees Departments Key Constraint (time constraint) At most one!!!

40. Database Management Systems 40 Raghu Ramakrishnan 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 ). u Every did value in Departments table must appear in a row of the Manages table (with a non-null ssn value!) lot name dname budgetdid since name dname budgetdid since Manages since Departments Employees ssn Works_In Total w/key constraint Partial Total

41. Database Management Systems 41 Raghu Ramakrishnan lot name dname budgetdid since name dname budgetdid since Manages since Departments Employees ssn Works_In Total w/key constraint Total What are the policies behind this ER model?

42. Database Management Systems 42 Raghu Ramakrishnan lot name dname budgetdid since name dname budgetdid since Manages since Departments Employees ssn Works_In Total w/key constraint Partial Total dname budgetdid since lot name ssn Manages Employees Departments Works_In Any Difference?

43. Database Management Systems 43 Raghu Ramakrishnan Weak Entities vs. Owner Entities v A weak entity can be identified uniquely only by considering the primary key of another ( owner ) entity. –Owner entity set and weak entity set must participate in a one-to-many relationship set (1 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 Weak Entity Identifying Relationship Primary Key for weak entity

44. Database Management Systems 44 Raghu Ramakrishnan dname budget did name Departments ssn lot Employees Works_In3 Duration from to Ternary Relationship lot dname budget did since name Works_In Departments Employees ssn Why?

45. Database Management Systems 45 Raghu Ramakrishnan ISA (`is a’) Hierarchies Contract_Emps name ssn Employees lot hourly_wages ISA Hourly_Emps contractid hours_worked v As in C++, or other PLs, attributes are inherited. v If we declare A ISA B, every A entity is also considered to be a B entity. v Overlap constraints : Can Joe be an Hourly_Emps as well as a Contract_Emps entity? ( Allowed/disallowed ) v Covering constraints : Does every Employees entity also have to be an Hourly_Emps or a Contract_Emps entity? (Yes/no) v Reasons for using ISA : –To add descriptive attributes specific to a subclass. –To identify entitities that participate in a relationship.

46. Database Management Systems 46 Raghu Ramakrishnan Aggregation v Used when we have to model a relationship involving (entitity sets and) a relationship set. – Aggregation allows us to treat a relationship set as an entity set for purposes of participation in (other) relationships. –Monitors mapped to table like any other relationship set. budget did pid started_on pbudget dname until Departments Projects Sponsors Employees Monitors lot name ssn Aggregation

47. Database Management Systems 47 Raghu Ramakrishnan

48. Database Management Systems 48 Raghu Ramakrishnan Real Database Design v Build an ER Diagram for the following information: –Walmart Stores u Store Id, Address, Phone # –Products u Product Id, Description, Price –Manufacturers u Name, Address, Phone # –Walmart Stores carry products u Amount in store –Manufacturers make products u Amount in factory/warehouses

49. Database Management Systems 49 Raghu Ramakrishnan Conceptual Design Using the ER Model v 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 ?

50. Database Management Systems 50 Raghu Ramakrishnan Entity vs. Attribute v 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: u If we have several addresses per employee, address must be an entity (since attributes cannot be set-valued). u 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).

51. Database Management Systems 51 Raghu Ramakrishnan Entity vs. Attribute (Contd.) v Works_In2 does not allow an employee to work in a department for two or more periods. v 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. name Employees ssn lot Works_In2 from to dname budget did Departments dname budget did name Departments ssn lot Employees Works_In3 Duration from to

52. Database Management Systems 52 Raghu Ramakrishnan Entity vs. Relationship v First ER diagram OK if a manager gets a separate discretionary budget for each dept. –Redundancy of dbudget, which is stored for each dept managed by the manager. –Misleading: suggests dbudget tied to managed dept. v What if a manager gets a discretionary budget that covers all managed depts? Manages2 name dname budget did Employees Departments ssn lot dbudget since Employees since name dname budget did Departments ssn lot Mgr_Appts Manages3 dbudget apptnum

53. Database Management Systems 53 Raghu Ramakrishnan Binary vs. Ternary Relationships v If each policy is owned by just 1 employee: –Key constraint on Policies would mean policy can only cover 1 dependent! age pname Dependents Covers name Employees ssn lot Policies policyid cost Beneficiary age pname Dependents policyid cost Policies Purchaser name Employees ssn lot Bad design Better design *

54. Database Management Systems 54 Raghu Ramakrishnan dname budget did name Departments ssn lot Employees Works_In3 Duration from to Binary vs. Ternary Relationships (Contd.) dname budget did name Departments ssn lot Employees Works_In3 from to

55. Database Management Systems 55 Raghu Ramakrishnan Binary vs. Ternary Relationships (Contd.) v 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 set Parts, Departments and Suppliers, and has descriptive attributes 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?

56. Database Management Systems 56 Raghu Ramakrishnan Summary of Conceptual Design v Conceptual design follows requirements analysis, –Yields a high-level description of data to be stored v ER model popular for conceptual design –Constructs are expressive, close to the way people think about their applications. v Basic constructs : entities, relationships, and attributes (of entities and relationships). v Some additional constructs : weak entities, ISA hierarchies, and aggregation. v Note: There are many variations on ER model.

57. Database Management Systems 57 Raghu Ramakrishnan 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.

58. Database Management Systems 58 Raghu Ramakrishnan 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 n-ary relationship, whether or not to use ISA hierarchies, and whether or not to use aggregation. v Ensuring good database design: resulting relational schema should be analyzed and refined further. FD information and normalization techniques are especially useful.

59. Database Management Systems 59 Raghu Ramakrishnan Erwin ER Modeling Tool v http://www.cai.com/products/alm/erwin.htm v Demo ERwin and it’s capabilities –Open sample movies model u Erwin_3.5.2/models –Build Example 2 using ERwin

60. Database Management Systems 60 Raghu Ramakrishnan Homework Assignment v Problem 2.4 at the end of Chapter 2 –Pages 53 v Due Next Thursday: Hard copy to instructor v Format for homework: name, ID.

61. Database Management Systems 61 Raghu Ramakrishnan Homework Assignment lot name dname budgetdid since name dname budgetdid since Manages since Departments Employees ssn Works_In Total w/key constraint Partial Total age pname DependentsPolicy cost Key/total partial