Chapter 2 The Relational Model

2-2 In This Chapter You Will Learn   What a data model is and what its uses are   The terminology of the relational model   How tables are used to represent data   Properties of database tables   How to identify candidate, primary, and foreign keys   The meaning of entity integrity （實體完整限制式） and referential integrity （參照完整限制式）

2-3 What Is A Data Model?   An integrated collection of concepts for describing data, relationships between data, and constraints on the data.   A data model attempts to represent the operation of a company or part of the company, that you wish to model.   It provides the essential concepts and notations that allow database designers and end-users to communicate their understanding of the company unambiguously and accurately.

2-4 Three Components Of A Data Model   A Structure Part   How the database is to be constructed   A Manipulative Part   Types of data operations that are allowed   Updating or retrieving data   Changing the structure of the database   A Set Of Integrity Rules   Ensures that the data is accurate

2-5 Terminology of Relational Model   Relation （關聯、關係）   Table with columns and rows   Columns ~ Attributes   Rows ~ Individual records   We use tables to hold information about the objects that we want to represent in the database.   Attribute （屬性）   Named column of a table   Can appear in any order, and the table will still be the same

2-6   A case example : StayHome Video Rental Company   The information on branches ( 分店 ) is represented by the Branch relation, with columns for attributes branchNo, street, city, state, zipCode, and mgrStaffNo.   The information on staff is represented by the Staff relation, with columns for attributes staffNo, name, position, salary, and branchNo. Terminology of Relational Model

2-7 An example of the Branch and Staff relations

2-8   Domain （定義域）   A set of allowable (possible) values for one or more attributes   Every attribute in a relational database is associated with a domain   May be distinct for each attribute. Or, two or more attributes may be associated with the same domain Terminology of Relational Model

2-9 Domain Example (For Some Attributes of Branch Relation

2-10   Tuple （值組）   A record of a relation   In the Staff relation, each record contains five values, one for each attribute.   A Relational Database   A collection of normalized tables with distinct table names. Terminology of Relational Model

2-11 Seven Properties of Relational Tables 1. 1.Table name is distinct from all other table names in the database Each column has a distinct name in a table Each cell of a table contains exactly one value.   To store several telephone number for a single branch in a single cell is wrong (No repeating groups.)   A table satisfying this property is said to be normalized or in first normal ( 正規化 ) form.

2-12 Seven Properties of Relational Tables 4. 4.Values of a column are all from the same domain 5. 5.Each record is distinct.   There are no duplicate records Order of columns has no significance (To users and programmers) 7. 7.Order of records has no significance. (To users and programmers)

2-13 Relational Keys   Candidate key （候選鍵）   Primary key （主鍵）   Foreign key （外部鍵、外來鍵、外鍵）

2-14 Candidate Key （候選鍵）   A column, or a set of columns, that uniquely identifies a record within a table.   Uniqueness （唯一性） : In a table, the value of the candidate key uniquely identifies a record   Irreducibility （不可減少性，最小性 ） : No proper subset of the candidate key has the uniqueness property   The key cannot be made smaller or simpler for unique identification   There may be many candidate keys for a table   When a key consists of more than one column, we call it a composite key （組合鍵）.

2-15 Candidate Key （候選鍵） Can city be a candidate key? Can zipCode be a candidate key? Can branchNo be a candidate key? Can mgrStaffNo be a candidate key? Branch relation

2-16 Candidate Key （候選鍵） Can actorNo be a candidate key? Role Table The characters played by actors in videos Can catalogNo be a candidate key? actorNocatalogNo? actorNo + catalogNo? actorNo+ character? actorNo + character?

2-17 Keys Primary Key （主鍵）  The candidate key selected to identify records uniquely for a given table.  A table always has a primary key.  The Worst Case: The entire set of columns serve as the primary key

2-18 Rules To Select Primary Key   With minimal number of attributes   Less likely to have its values changed (or never changed)   Less likely to lose uniqueness in the future   Always have values   With fewest characters (for those with textual attribute)   With the smallest maximum value (for numerical attribute)   Easiest to use from the users’ point of view

2-19 Foreign Key （外部鍵、外索引）   A column, or set of columns, within one table that matches the primary key of some (possibly same) table.   When a column appears in more than one table, its appearance represents a relationship between the records of the two tables

2-20 Relational Keys Parent/Home Table Primary Key Foreign Key Child Table

2-21 Functions of Keys 1.Who is the manager of the branch at Portland, OR ? 2.Who are the managers of the branch at Seattle, WA ? 3. Who is Sally Adams’s manager ?

2-22 Relational Keys of COMPANY Database (PK and FK)

2-23 Representing Relational Databases (The StayHome Database) Branch ( branchNo, street, city, state, zipCode, mgStaffNo) Staff ( staffNo, name, position, salary, branchNo) Video ( catalogNo, title, category, dailyRental, price, directorNo) Director ( directorNo, directorName) Actor ( actorNo, catalogNo) Role ( actorNo, catalogNo, character) Member ( memberNo, fName, lName, address) Registration ( branchNo, memberNo, staffNo, dateJoined) RentalAgreement ( rentalNo, dateOut, dateReturn, memberNo, videoNo) VideoForRent ( videoNo, available, catalogNo, branchNo) data example data example

2-24 Data example of StayHome DB (1)

2-25 Data example of StayHome DB (2)

2-26 Relational Integrity （關聯完整限制式、關聯的完整性）   Ensures that the data of a database is accurate   Constraints that apply to the entire database   Entity Integrity （實體完整限制式）   Referential Integrity （參照完整限制式）   Before defining these two constraints, let’s look at   Null ( 虛值）   Business Rules

2-27 Null （虛值、空值）   Represents a value for a column that is currently unknown or is not applicable ( 不適用 ) for a record.   Deals with incomplete or exceptional data.   Examples   A branch temporarily without a manager

2-28 Business Rules （企業規則）   Specify additional constraints that the data must satisfy.   Represented in the form of rules that define or constrain some operations of the organization.   Example   A member can only rent a maximum of 10 videos at any one time

2-29 Entity Integrity （實體完整限制式、實體的完整性）   Applies to the primary key of a base table.   In a base table, no column of a primary key can be null.   A base table is a named table whose records are physically stored in the database.   Note: A view is a ‘virtual table’ that does not actually exist in the database but is generated by the DBMS from some underlying base tables dynamically.

2-30 Referential Integrity （參照完整限制式、參照的完整性）   If foreign key exists in a table,   either the foreign key value of a record in the table must match a candidate key value of some record in its parent/home table,   or the foreign key value must be wholly null.   Example:   Not possible to create a staff record with branch number B300 (since B300 does not exist.)   Can create a new staff record with a null branch number if null value is allowed