1. © 2002 by Prentice Hall 1 The Relational Model David M. Kroenke Database Concepts 1e Chapter 2 2

2. © 2002 by Prentice Hall2 Chapter Objectives Learn the conceptual foundation of the relational model Understand how relations differ from nonrelational tables Learn basic relational terminology Learn the meaning and importance of keys, foreign keys, and related terminology

3. © 2002 by Prentice Hall3 Chapter Objectives (continued) Understand how foreign keys represent relationships Learn the purpose and use of surrogate keys Learn the meaning of functional dependencies Understand two basic relational design principles

4. © 2002 by Prentice Hall4 A Relation A relation is a two-dimensional table that has specific characteristics The table dimensions, like a matrix, consist of rows and columns

5. © 2002 by Prentice Hall5 Characteristics of a Relation Rows contain data about an entity Columns contain data about attributes of the entity Cells of the table hold a single value All entries in a column are of the same kind Each column has a unique name The order of the columns is unimportant The order of the rows is unimportant No two rows may be identical

6. © 2002 by Prentice Hall6 A Sample Relation EmployeeNumberFirstNameLastName 100MaryAbermany 700JerryCaldera 300AleaCopley 40MuruganJacksoni

7. © 2002 by Prentice Hall7 A Nonrelation Example EmployeeNumberPhoneLastName 100335-6421, 454-9744 Abermany 700215-7789Caldera 300610-9850Copley 40299-9090Jacksoni Cells of the table hold a single value

8. © 2002 by Prentice Hall8 EmployeeNumberPhoneLastName 100335-6421Abermany 700215-7789Caldera 300610-9850Copley 100335-6421Abermany 40299-9090Jacksoni No two rows may be identical A Nonrelation Example

9. © 2002 by Prentice Hall9 Terminology TableRowColumn File or DatafileRecordField RelationTupleAttribute Synonyms…

10. © 2002 by Prentice Hall10 A Key A key is one or more columns of a relation that is used to identify a row

11. © 2002 by Prentice Hall11 Uniqueness of Keys Unique KeyNonunique Key Data value is unique for each row. Consequently, the key will uniquely identify a row. Data value may be shared among several rows. Consequently, the key will identify a set of rows.

12. © 2002 by Prentice Hall12 A Composite Key A composite key is a key that contains two or more attributes For a key to be unique, often it must become a composite key

13. © 2002 by Prentice Hall13 Composite Key Example To identify a family member, you need to know a FamilyID, a FirstName, and a Suffix (e.g., Jr.) The composite key is: FamilyID, FirstName, Suffix One needs to know the value of all three columns to uniquely identify an individual

14. © 2002 by Prentice Hall14 A Candidate Key A candidate key is called “candidate” because it is a candidate to become the primary key A candidate key is a unique key

15. © 2002 by Prentice Hall15 A Primary Key A primary key is one chosen candidate key If you know the value of the primary key, you will be able to uniquely identify a single row

16. © 2002 by Prentice Hall16 Table Relationships A table may be related to other tables For example –An Employee works in a Department –A Manager controls a Project

17. © 2002 by Prentice Hall17 A Foreign Key To preserve relationships, you may need to create a foreign key A foreign key is a primary key from one table placed into another table The key is called a foreign key in the table that received the key

18. © 2002 by Prentice Hall18 Foreign Key Example Project ProjID ProjName MgrID Manager MgrID MgrName Foreign Key Primary Key

19. © 2002 by Prentice Hall19 Foreign Key Example Department DeptID DeptName Location Employee EmpID DeptID EmpName Foreign Key Primary Key

20. © 2002 by Prentice Hall20 Referential Integrity Referential integrity states that every value of a foreign key must match a value of the primary key For example –If EmpID = 4 has a DeptID = 7 (a foreign key). A Department with DeptID = 7 must exist and no other Department may have DeptID = 7

21. © 2002 by Prentice Hall21 Referential Integrity Another perspective… (The value of the Foreign Key EmployeeID) in EQUIPMENT must exist in (as a Primary Key value EmployeeID) in EMPLOYEE

22. © 2002 by Prentice Hall22 A Surrogate Key A Surrogate Key is a new key that takes the place of the Primary Key A Surrogate Key is typically used in place of a composite primary key It is clearer and easier to deal with one column, rather than a set of columns

23. © 2002 by Prentice Hall23 Surrogate Key Example Family Member Primary Key is FamilyID, FirstName, Suffix. It is easier to create a surrogate key of FamMemberID. This would be a unique key Referential Integrity… (FamilyID, FirstName, Suffix) in School must exist in (FamilyID, FirstName, Suffix) in FamilyMember Alternatively… (FamMemberID) in School must exist in (FamMemberID) in FamilyMember

24. © 2002 by Prentice Hall24 Functional Dependency The value of one column may determine the value of another column in the same table Illustration… –The price of one cookie can determine the price of a box of 12 cookies (CookiePrice, Qty)BillAmount

25. © 2002 by Prentice Hall25 Candidate/Primary Keys and Functional Dependency By definition… A candidate key of a relation will functionally determine all other columns in the row Likewise, by definition… A primary key of a relation will functionally determine all other columns in the row

26. © 2002 by Prentice Hall26 Primary Key and Functional Dependency Example (EmployeeID) (EmpLastName, EmpAddr, EmpPhone) (ProjectID)(ProjectName, StartDate)

27. © 2002 by Prentice Hall27 Normalization Normalization is a process of analyzing a relation to ensure it is well formed More specifically, if a relation is normalized, rows can be added, removed, or updated without creating exceptions

28. © 2002 by Prentice Hall28 Stages of Normalization Relations are tested for various conditions to ensure normalization At the conclusion of specific sets of tests, a relation is said to be in a “Normal Form” There are several Normal Forms –First Normal Form, Second Normal Form, Third Normal Form, Boyce-Codd Normal Form, Fourth Normal Form, Fifth Normal Form, and Domain/Key Normal Form

29. © 2002 by Prentice Hall29 Un-Normalized Example (StudentID) (StudentName, DormName, DormCost) (DormName)(DormCost) Then DormCost should be placed into its own relation However, if… (StudentID) (StudentName, DormName) Resulting in… (DormName)(DormCost)

30. © 2002 by Prentice Hall30 Un-Normalized Example (AttorneyID, ClientID) (ClientName, MeetingDate, Duration) (ClientID)(ClientName) Then ClientName should be placed into its own relation However, if… Resulting in… (ClientID)(ClientName) (AttorneyID, ClientID) (MeetingDate, Duration)

31. © 2002 by Prentice Hall31 The Null Value A Null value means that no data was entered This is different from a zero, space character, or tab character

32. © 2002 by Prentice Hall32 The Problem of Null Values A Null is often ambiguous. It could mean… –The column value is not appropriate for the specific row –The column value is not decided –The column value is unknown Each may have entirely different implications

33. © 2002 by Prentice Hall 33 The Relational Model David M. Kroenke Database Concepts 1e Chapter 2 2