1. Concepts of Database Management, Fifth Edition Chapter 6: Database Design 2: Design Methodology

2. 2 Concepts of Database Management, 5t h Edition Objectives u Discuss the general process and goals of database design u Define user views and explain their function u Define database design language and use it to document database designs u Create an entity-relationship diagram to visually represent a database design u Present a methodology for database design at the information level and view examples illustrating this methodology

3. 3 Concepts of Database Management, 5t h Edition Objectives u Explain the physical-level design process u Discuss top-down and bottom-up approaches to database design and examine the advantages and disadvantages of both methods u Use a survey form to obtain information from users prior to beginning the database design process u Review existing documents to obtain information prior to beginning the database design

4. 4 Concepts of Database Management, 5t h Edition Objectives u Discuss special issues related to implementing one-to-one relationships and many-to-many relationships involving more than two entities u Discuss entity subtypes and their relationships to nulls u Learn how to avoid potential problems when merging third normal form relations u Examine the entity-relationship model for representing and designing databases

5. 5 Concepts of Database Management, 5t h Edition Database Design u Information-level Design – building a database that satisfies the organization’s requirements as cleanly as possible u Physical-level design – designers transform the information-level design into a design for the specific DBMS used by the organization u User Views – requirements necessary to support a particular user’s operations u Cumulative design – supports all user views encountered so far in the design process

6. 6 Concepts of Database Management, 5t h Edition Information-Level Design Methodology u Information-level Design Methodology l Represent user view as collection of tables l Normalize these tables l Identify all keys l Merge the result into design

7. 7 Concepts of Database Management, 5t h Edition Represent User View as Collection of Tables Step 1: Determine entities involved and create separate table for each type Step 2: Determine primary key for each table Step 3: Determine properties for each entities Step 4: Determine relationships among entities

8. 8 Concepts of Database Management, 5t h Edition Types of Relationships u One-to-Many Relationships – created by including the primary key of the “one” table as a foreign key in the “many” table u Many-to-Many Relationships – created with a new table whose primary key is the combination of the primary keys of the original tables u One-to-One Relationship – created with a single relationship from one record in a table to a single record in another table

9. 9 Concepts of Database Management, 5t h Edition Normalize the Tables u Represent all keys l Primary, alternate, secondary, foreign u Database Design Language (DBDL) l Mechanism for representing tables and keys DBDL Example Figure 6.1

10. 10 Concepts of Database Management, 5t h Edition Types of Primary Keys u Three types of primary keys used in database design 1. Natural key (logical key or intelligent key) – consists of a column that uniquely identifies an entity 2. Artificial key – column created for an entity to serve solely as the primary key; visible to users 3. Surrogate key (synthetic key) – system-generated primary key usually hidden from users

11. 11 Concepts of Database Management, 5t h Edition DBDL Notation u Table name followed by columns in parentheses l Primary key column(s) underlined u AK identifies alternate keys u SK identifies secondary keys u FK identifies foreign keys

12. 12 Concepts of Database Management, 5t h Edition Entity-Relationship Diagrams u Diagram that visually represents database structure l Rectangle represents each entity in the E-R diagram u Primary key for each entity appears above the line in the rectangle for each entry

13. 13 Concepts of Database Management, 5t h Edition Entity-Relationship Diagrams u Other columns that comprise each entity appear below the line within each rectangle u The letters AK, SK, and FK appear in parentheses following the alternate key, secondary key, and foreign key respectively u For each foreign key, there is a line leading from the rectangle that corresponds to the table being identified to the rectangle that corresponds to the table containing the foreign key

14. 14 Concepts of Database Management, 5t h Edition Figure 6.2: Entity-Relationship Diagrams

15. 15 Concepts of Database Management, 5t h Edition Merge the Result into the Design u As soon as Steps 1 through 3 for a given user view have been completed, the results can be merged into the cumulative design l If the working view is the first user view, the cumulative design will be identical to the design for the first user l Otherwise, all the tables for this user with those that are currently in the cumulative design

16. 16 Concepts of Database Management, 5t h Edition Merge the Result into the Design (con’t) u Next, you combine tables that have the same primary key to form a new table l New table has the same primary key as those tables you have combined l New table also contains all the columns from both tables l In the case of duplicate columns, you remove all but one copy of the column

17. 17 Concepts of Database Management, 5t h Edition Figure 6.4: User View Examples View #1: Sales Rep View Rep (RepNum, LastName, FirstName, Street, City, State, Zip, Commission, Rate)

18. 18 Concepts of Database Management, 5t h Edition View #2: Customer View Figure 6.5: User View Examples

19. 19 Concepts of Database Management, 5t h Edition Entities u Independent entity - an entity that does not require a relationship to another entity for identification u Dependent entity - an entity that does require a relationship to another entity for identification

20. 20 Concepts of Database Management, 5t h Edition Physical-Level Design u Undertaken after information-level design completion u Most DBMSs support primary, candidate, secondary, and foreign keys u DB programmers must include logic to ensure the uniqueness of primary keys and enforce other conditions

21. 21 Concepts of Database Management, 5t h Edition Top-Down vs. Bottom-Up u Bottom-up l Design starts at low level l Specific user requirements drive design process u Top-down l Begins with general database that models overall enterprise l Refines the model until design is achieved

22. 22 Concepts of Database Management, 5t h Edition Survey Form u Used to collect information from users u Must contain particular elements l Entity information l Attribute information l Relationships l Functional dependencies l Processing information

23. 23 Concepts of Database Management, 5t h Edition Existing Documents u Aid in collecting user requirements u Collect information similar to that collected with survey forms l Entity information l Attribute information l Relationships l Functional dependencies l Processing information

24. 24 Concepts of Database Management, 5t h Edition Figure 6.14: Example Invoice

25. 25 Concepts of Database Management, 5t h Edition Figure 6.15: List of Possible Attributes

26. 26 Concepts of Database Management, 5t h Edition Figure 6.16: Tentative List of Functional Dependencies

27. 27 Concepts of Database Management, 5t h Edition Figure 6.17: Revised List of Functional Dependencies

28. 28 Concepts of Database Management, 5t h Edition Figure 6.18: Tentative List of Entities

29. 29 Concepts of Database Management, 5t h Edition Figure 6.19: Expanded List of Entities

30. 30 Concepts of Database Management, 5t h Edition One-to-One Relationship Considerations u Implementing a one-to-one relationship by simply including the primary key of each table as a foreign key in the other table l A problem is that there is no guarantee that the information will match

31. 31 Concepts of Database Management, 5t h Edition One-to-One Relationship Considerations (con’t) u One possible solution is to create a single table l Although workable, two features are not particularly attractive: 1. Combines columns of two different entities into a single table 2. If possible for one entity to exist without the other

32. 32 Concepts of Database Management, 5t h Edition One-to-One Relationship Considerations (con’t) u Better solution l Create two separate tables for customers and sales reps and to include the primary key of one of them as a foreign key in the other l This foreign key would also be designated as an alternate key

33. 33 Concepts of Database Management, 5t h Edition Figure 6.20: 1:1 Relationship Considerations Include primary key of each table as foreign key in the other

34. 34 Concepts of Database Management, 5t h Edition Figure 6.21: 1:1 Relationship Considerations Implementation when information does not match

35. 35 Concepts of Database Management, 5t h Edition Figure 6.22: 1:1 Relationship Considerations Implemented in a single table

36. 36 Concepts of Database Management, 5t h Edition Figure 6.23: 1:1 Relationship Considerations 1:1 relationship implemented by including primary key of one table as foreign key (and alternative key) in the other

37. 37 Concepts of Database Management, 5t h Edition Many-to-Many Relationship Considerations u Complex issues arise when more than two entities are related in a many-to-many relationship u Many-to-many-to-many relationship – involves multiple entities u Crucial issue in making the determination between a single many-to-many-to-many relationship and two (or three) many-to-many relationships is the independence

38. 38 Concepts of Database Management, 5t h Edition Figure 6.24: M:M Relationship Considerations Sample Sales Data

39. 39 Concepts of Database Management, 5t h Edition Figure 6.25: M:M Relationship Considerations Result obtained by splitting Sales table into three tables

40. 40 Concepts of Database Management, 5t h Edition Figure 6.26: M:M Relationship Considerations Result obtained by joining three tables--2 rows are in error. Must be converted to 4NF

41. 41 Concepts of Database Management, 5t h Edition Nulls u Null - a special value u Null - actually represents the absence of a value in a field u Nulls - used when a value is either unknown or inapplicable

42. 42 Concepts of Database Management, 5t h Edition Nulls are absence of values Figure 6.27: Table Split To Avoid Nulls

43. 43 Concepts of Database Management, 5t h Edition Figure 6.28: Sample DBDL

44. 44 Concepts of Database Management, 5t h Edition Entity Subtypes u Entity subtype – table that is a subtype of another table u Incomplete category – records that do not fall into the subtype u Complete categories – all records fall into the categories

45. 45 Concepts of Database Management, 5t h Edition Figure 6.29: Entity Subtypes

46. 46 Concepts of Database Management, 5t h Edition Figure 6.32: Two Entity Subtypes— Incomplete Categories

47. 47 Concepts of Database Management, 5t h Edition Figure 6.33: Two Entity Subtypes— Complete Categories

48. 48 Concepts of Database Management, 5t h Edition Avoiding Problems with 3NF When Merging Tables u When combining third normal form tables, the result need not be in third normal form u To avoid the problem of creating a table that is not in third normal form, be cautious when representing user views u If you always attempt to determine whether determinants exist and include them in the tables, the problem is often avoided

49. 49 Concepts of Database Management, 5t h Edition Entity-Relationship Model u An approach to representing data in a database u Entities are drawn as rectangles u Relationships are drawn as diamonds with lines connecting the entities involved in relationships

50. 50 Concepts of Database Management, 5t h Edition Figure 6.34: One-to-many relationship

51. 51 Concepts of Database Management, 5t h Edition Figure 6.35: Many-to-Many Relationship

52. 52 Concepts of Database Management, 5t h Edition Figure 6.36: Many-to-Many Relationship

53. 53 Concepts of Database Management, 5t h Edition Figure 6.37: One-to-Many Relationship

54. 54 Concepts of Database Management, 5t h Edition Figure 6.38: Many-to- Many Relationship with Attributes

55. 55 Concepts of Database Management, 5t h Edition Composite Entity u Composite entity - an entity that exists to implement a many-to-many relationship l Essentially both an entity and a relationship l Represented in an E-R diagram by a diamond within a rectangle

56. 56 Concepts of Database Management, 5t h Edition Figure 6.39: Composite Entity

57. 57 Concepts of Database Management, 5t h Edition Figure 6.40: Complete E-R Diagram

58. 58 Concepts of Database Management, 5t h Edition E-R Diagram with an Existence Dependency and Weak Entity u Existence dependency - when the existence of one entity depends on the existence of another related entity u Indicate an existence dependency by placing an “E” in the relationship diamond u Weak entity - an entity that depends on another entity for its own existence u A double rectangle encloses a weak entity

59. 59 Concepts of Database Management, 5t h Edition Figure 6.41: E-R Diagram with an Existence Dependency and Weak Entity

60. 60 Concepts of Database Management, 5t h Edition Cardinality u Cardinality - the number of items that must be included in a relationship u An entity in a relationship with minimum cardinality of zero plays an optional role in the relationship u An entity with a minimum cardinality of one plays a mandatory role in the relationship

61. 61 Concepts of Database Management, 5t h Edition Figure 6.43: E-R Diagram that Represents Cardinality

62. 62 Concepts of Database Management, 5t h Edition Summary u Database design is a two-part process of determining an appropriate database structure to satisfy a given set of requirements u A user view is the set of necessary requirements to support a particular user’s operations u A database design is represented in a language called Database Design Language (DBDL) u Designs can be represented pictorially using entity-relationship (E-R) diagrams

63. 63 Concepts of Database Management, 5t h Edition Summary u Advantages to both top-down and bottom-up approaches u Survey form is useful for documenting the information gathered for database design process u One-to-One and One-to-Many relationships require attention to primary keys u Entity-relationship (E-R) model is a method of representing the structure of a database using an E-R diagram