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

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

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

3 Objectives Explain the physical-level design process Discuss top-down and bottom-up approaches to database design and examine the advantages and disadvantages of both methods Use a survey form to obtain information from users prior to beginning the database design process Review existing documents to obtain information prior to beginning the database design 3

4 Objectives Discuss special issues related to implementing one-to-one relationships and many-to-many relationships involving more than two entities Discuss entity subtypes and their relationships to nulls Learn how to avoid potential problems when merging third normal form relations Examine the entity-relationship model for representing and designing databases 4

5 Database Design Information-level Design building a database that satisfies the organization s requirements as cleanly as possible Physical-level design designers transform the information-level design into a design for the specific DBMS used by the organization User Views requirements necessary to support a particular user s operations Cumulative design supports all user views encountered so far in the design process 5

6 Information-Level Design Methodology Information-level Design Methodology Represent user view as collection of tables Normalize these tables Identify all keys Merge the result into design 6

7 Represent User View as Collection of Tables Step 1: Step 2: Step 3: Step 4: Determine entities involved and create separate table for each type Determine primary key for each table Determine properties for each entities Determine relationships among entities 7

8 Types of Relationships One-to-Many Relationships created by including the primary key of the one table as a foreign key in the many table Many-to-Many Relationships created with a new table whose primary key is the combination of the primary keys of the original tables One-to-One Relationship created with a single relationship from one record in a table to a single record in another table 8

9 Normalize the Tables Represent all keys Primary, alternate, secondary, foreign Database Design Language (DBDL) Mechanism for representing tables and keys DBDL Example Figure 6.1 9

10 Types of Primary Keys 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 10

11 DBDL Notation Table name followed by columns in parentheses Primary key column(s) underlined AK identifies alternate keys SK identifies secondary keys FK identifies foreign keys 11

12 Entity-Relationship Diagrams Diagram that visually represents database structure Rectangle represents each entity in the E-R diagram Primary key for each entity appears above the line in the rectangle for each entry 12

13 Entity-Relationship Diagrams Other columns that comprise each entity appear below the line within each rectangle The letters AK, SK, and FK appear in parentheses following the alternate key, secondary key, and foreign key respectively 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 13

14 Figure 6.2: Entity-Relationship Diagrams 14

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

16 Merge the Result into the Design (con t) Next, you combine tables that have the same primary key to form a new table New table has the same primary key as those tables you have combined New table also contains all the columns from both tables In the case of duplicate columns, you remove all but one copy of the column 16

17 Figure 6.4: User View Examples View #1: Sales Rep View Rep (RepNum, LastName, FirstName, Street, City, State, Zip, Commission, Rate) 17

18 Figure 6.5: User View Examples View #2: Customer View 18

19 Entities Independent entity - an entity that does not require a relationship to another entity for identification Dependent entity - an entity that does require a relationship to another entity for identification 19

20 Physical-Level Design Undertaken after information-level design completion Most DBMSs support primary, candidate, secondary, and foreign keys DB programmers must include logic to ensure the uniqueness of primary keys and enforce other conditions 20

21 Top-Down vs. Bottom-Up Bottom-up Design starts at low level Specific user requirements drive design process Top-down Begins with general database that models overall enterprise Refines the model until design is achieved 21

22 Survey Form Used to collect information from users Must contain particular elements Entity information Attribute information Relationships Functional dependencies Processing information 22

23 Existing Documents Aid in collecting user requirements Collect information similar to that collected with survey forms Entity information Attribute information Relationships Functional dependencies Processing information 23

24 Figure 6.14: Example Invoice 24

25 Figure 6.15: List of Possible Attributes 25

26 Figure 6.16: Tentative List of Functional Dependencies 26

27 Figure 6.17: Revised List of Functional Dependencies 27

28 Figure 6.18: Tentative List of Entities 28

29 Figure 6.19: Expanded List of Entities 29

30 One-to-One Relationship Considerations Implementing a one-to-one relationship by simply including the primary key of each table as a foreign key in the other table A problem is that there is no guarantee that the information will match 30

31 One-to-One Relationship Considerations (con t) One possible solution is to create a single table 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 31

32 One-to-One Relationship Considerations (con t) Better solution 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 This foreign key would also be designated as an alternate key 32

33 Figure 6.20: 1:1 Relationship Considerations Include primary key of each table as foreign key in the other 33

34 Figure 6.21: 1:1 Relationship Considerations Implementation when information does not match 34

35 Figure 6.22: 1:1 Relationship Considerations Implemented in a single table 35

36 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 36

37 Many-to-Many Relationship Considerations Complex issues arise when more than two entities are related in a many-to-many relationship Many-to-many-to-many relationship involves multiple entities 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 37

38 Figure 6.24: M:M Relationship Considerations Sample Sales Data 38

39 Figure 6.25: M:M Relationship Considerations Result obtained by splitting Sales table into three tables 39

40 Figure 6.26: M:M Relationship Considerations Result obtained by joining three tables--2 rows are in error. Must be converted to 4NF 40

41 Nulls Null - a special value Null - actually represents the absence of a value in a field Nulls - used when a value is either unknown or inapplicable 41

42 Figure 6.27: Table Split To Avoid Nulls Nulls are absence of values 42

43 Figure 6.28: Sample DBDL 43

44 Entity Subtypes Entity subtype table that is a subtype of another table Incomplete category records that do not fall into the subtype Complete categories all records fall into the categories 44

45 Figure 6.29: Entity Subtypes 45

46 Figure 6.32: Two Entity Subtypes Incomplete Categories 46

47 Figure 6.33: Two Entity Subtypes Complete Categories 47

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

49 Entity-Relationship Model An approach to representing data in a database Entities are drawn as rectangles Relationships are drawn as diamonds with lines connecting the entities involved in relationships 49

50 Figure 6.34: One-to-many relationship 50

51 Figure 6.35: Many-to-Many Relationship 51

52 Figure 6.36: Many-to-Many Relationship 52

53 Figure 6.37: One-to-Many Relationship 53

54 Figure 6.38: Many-to- Many Relationship with Attributes 54

55 Composite Entity Composite entity - an entity that exists to implement a many-to-many relationship Essentially both an entity and a relationship Represented in an E-R diagram by a diamond within a rectangle 55

56 Figure 6.39: Composite Entity 56

57 Figure 6.40: Complete E-R Diagram 57

58 E-R Diagram with an Existence Dependency and Weak Entity Existence dependency - when the existence of one entity depends on the existence of another related entity Indicate an existence dependency by placing an E in the relationship diamond Weak entity - an entity that depends on another entity for its own existence A double rectangle encloses a weak entity 58

59 Figure 6.41: E-R Diagram with an Existence Dependency and Weak Entity 59

60 Cardinality Cardinality - the number of items that must be included in a relationship An entity in a relationship with minimum cardinality of zero plays an optional role in the relationship An entity with a minimum cardinality of one plays a mandatory role in the relationship 60

61 Figure 6.43: E-R Diagram that Represents Cardinality 61

62 Summary Database design is a two-part process of determining an appropriate database structure to satisfy a given set of requirements A user view is the set of necessary requirements to support a particular user s operations A database design is represented in a language called Database Design Language (DBDL) Designs can be represented pictorially using entity-relationship (E-R) diagrams 62

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

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