Chapter 3: Modeling Data in the Organization 註 : 於 11版為Chapter 2 楊立偉教授 台灣大學 管系 2015 Fall 1
Business Rules n Statements that define or constrain some aspect of the business 用來定義或限制 : Ex. 每人每學期修課總學分上限為 30學分 n Control/influence business behavior 控制或 影響行為 Ex. 避免學生超修行為 n Expressed in terms familiar to end users 以使用者熟悉的語言來表達 n Automated through DBMS software Ex. 修 課系統應自動擋掉超修行為 Chapter 3 2
A Good Business Rule Is: n n n n Declarative–what, not how 陳述清楚 Precise–clear, agreed-upon meaning 精確 Atomic–one statement 不可分割 Consistent–internally and externally 一致 Expressible–structured, natural language 可被結 構化的口語表達 Distinct–non-redundant 非贅述的 Business-oriented–understood by business people 可被該領域的人了解 Chapter 3 3
A Good Data Name Is: n n n n Related to business, not technical 非技術詞 Meaningful and self-documenting 看了就懂 Unique 獨一 Readable 可讀性高 Composed of words from an approved list 是可用列表中的語詞 Repeatable 適用多種狀況 Follows standard syntax 有固定命名方式 Chapter 3 4
E-R Model Constructs n Entities: 個體 n n n Entity instance – person, place, object, event, concept (often corresponds to a row in a table) Entity Type – collection of entities (often corresponds to a table) Relationships: 關係 Relationship instance – link between entities (corresponds to primary key-foreign key equivalencies in related tables) n Relationship type – category of relationship…link between entity types Attribute: 屬性 – property or characteristic of an entity or relationship type (often corresponds to a field in a table) n n Chapter 3 5
Sample E-R Diagram (Figure 3 -1) 1 Chapter 3 0 or 1 1…N 0…N 6
Basic E-R notation (Figure 3 -2) Entity symbols A special entity that is also a relationship Relationship degrees specify number of entity types involved Chapter 3 Attribute symbols Relationship cardinalities specify how many of each entity type is allowed 7
先找出 Entity What Should an Entity Be? n SHOULD BE: An object that will have many instances in the database n An object that will be composed of multiple attributes n An object that we are trying to model n n SHOULD NOT BE: A user of the database system n An output of the database system (e. g. , a report) n Chapter 3 8
Figure 3 -4 Example of inappropriate entities System user X 會計出納員 (這是操作的人) Inappropriate entities System output X 費用報告 (這是運算後的結果) Appropriate entities Chapter 3 9
Attributes n n Attribute–property or characteristic of an entity or relationship type Classifications of attributes: n n n Required versus Optional Attributes 是否必填 Simple versus Composite Attribute 複合屬性 Ex. 姓名 vs 姓+名 Single-Valued versus Multivalued Attribute 是否有多值 Stored versus Derived Attributes 是否為衍生屬性 Ex. 出生年 vs 年齡 Identifier Attributes 是否可供識別用 Ex. 學號 Chapter 3 10
Data Modeling Concepts: Attributes Attribute – a descriptive property or characteristic of an entity. Synonyms include element, property, and field. n Just as a physical student can have attributes, such as hair color, height, etc. , data entity has data attributes Compound attribute – an attribute that consists of other attributes. Synonyms in different data modeling languages are numerous: concatenated attribute, composite attribute, and data structure. 例如: 姓名屬性可由姓+名所組成 右方紅框內即為複合屬性 Chapter 3
Data Modeling Concepts: Data Type 資料型別 – 這個屬性可以裝哪種資料 Data type – a property of an attribute that identifies what type of data can be stored in that attribute. Representative Logical Data Types for Attributes Data Type NUMBER TEXT MEMO DATE TIME YES/NO VALUE Chapter 3 SET Logical Business Meaning Any number, real or integer. 實數或整數 A string of characters, inclusive of numbers. When numbers are included in a TEXT attribute, it means that we do not expect to perform arithmetic or comparisons with those numbers. 文字 Same as TEXT but of an indeterminate size. Some business systems require the ability to attach potentially lengthy notes to a give database record. 不定長度的文字 Any date in any format. Any time in any format. An attribute that can assume only one of these two values. → 布林 (boolean) A finite set of values. In most cases, a coding scheme would be established (e. g. , FR=Freshman, SO=Sophomore, JR=Junior,
Data Modeling Concepts: Domains 資料範圍 – 這個屬性裝哪些值才是合法 Domain – a property of an attribute that defines what values an attribute can legitimately take on. Representative Logical Domains for Logical Data Types Data Type Domain Examples NUMBER For integers, specify the range. 值域與小數 位數 For real numbers, specify the range and precision. {10 -99} {1. 000 -799. 999} TEXT Maximum size of attribute. Actual values usually infinite; however, users may specify certain narrative restrictions. 最大文字長度 Text(30) DATE Variation on the MMDDYYYY format. MMDDYYYY MMYYYY For AM/PM times: HHMMT Chapter 3 TIME
Data Modeling Concepts: Default Value 預設值 Default value – the value that will be recorded if a value is not specified by the user. Permissible Default Values for Attributes Default Value Interpretation A legal value For an instance of the attribute, if the user does from the domain not specify a value, then use this value. Examples 0 1. 00 NONE or NULL For an instance of the attribute, if the user does NONE not specify a value, then leave it blank. 空值或無 NULL 值 Required or NOT NULL For an instance of the attribute, require that the user enter a legal value from the domain. (This is used when no value in the domain is common enough to be a default but some value must be entered. ) 必填 或 不可為無值 Chapter 3 REQUIRED NOT NULL
Identifiers (Keys) n n n Identifier (Key)–an attribute (or combination of attributes) that uniquely identifies individual instances of an entity type Ex. 學號 or 系所+姓名 Simple versus Composite Identifier Candidate Identifier – an attribute that could be a key…satisfies the requirements for being an identifier Chapter 3 15
Characteristics of Identifiers n n Will not change in value 不會更改 Will not be null 不會無值 No intelligent identifiers (e. g. , containing locations or people that might change) 不 要包含可能更改的欄位 Ex. 系所+姓名 Substitute new, simple keys for long, composite keys 簡短單一最好 Chapter 3 16
Figure 3 -7 A composite attribute 地址 An attribute broken into component parts Figure 3 -8 Entity with multivalued attribute (Skill) and derived attribute (Years_Employed) 技能 Multivalued an employee can have more than one skill Chapter 3 年資 Derived from date employed and current date 17
Figure 3 -9 Simple and composite identifier attributes ID欄位 (或稱 Key鍵值) The identifier is boldfaced and underlined Chapter 3 18
Figure 3 -19 Simple example of time-stamping 歷史價格 This attribute is both multivalued and composite 注意大括號與小括號 Chapter 3 19
More on Relationships n Relationship Types vs. Relationship Instances n n Relationships can have attributes n n n The relationship type is modeled as lines between entity types…the instance is between specific entity instances These describe features pertaining to the association between the entities in the relationship Two entities can have more than one type of relationship between them (multiple relationships) Associative Entity – combination of relationship and entity 特殊關係所轉換成的entity Chapter 3 20
Figure 3 -10 Relationship types and instances a) Relationship type b) Relationship instances Chapter 3 21
Degree of Relationships n Degree of a relationship is the number of entity types that participate in it n Unary Relationship n. A relationship between different instances of the same entity is called a recursive relationship n Binary Relationship n Ternary Relationship Chapter 3 22
Degree of relationships – from Figure 3 -2 One entity related to another of the same entity type Ex. 朋友關係 Chapter 3 Entities of two different types related to each other Ex. 修課關係 Entities of three different types related to each other Ex. 作分派關係 23
Cardinality of Relationships n One-to-One n n One-to-Many n n Each entity in the relationship will have exactly one related entity An entity on one side of the relationship can have many related entities, but an entity on the other side will have a maximum of one related entity Many-to-Many n Entities on both sides of the relationship can have many related entities on the other side Chapter 3 24
Cardinality Constraints n n Cardinality Constraints—the number of instances of one entity that can or must be associated with each instance of another entity Minimum Cardinality 最小值 If zero, then optional n If one or more, then mandatory n n Maximum Cardinality 最大值 n The maximum number Chapter 3 25
Figure 3 -12 Examples of relationships of different degrees a) Unary relationships Chapter 3 26
Figure 3 -12 Examples of relationships of different degrees (cont. ) b) Binary relationships Chapter 3 27
Figure 3 -12 Examples of relationships of different degrees (cont. ) c) Ternary relationship 零件 廠商 倉庫 供應出貨關係 Note: a relationship can have attributes of its own Chapter 3 28
Figure 3 -17 Examples of cardinality constraints a) Mandatory cardinalities 必填 A patient history is recorded for one and only one patient Chapter 3 A patient must have recorded at least one history, and can have many 29
Figure 3 -17 Examples of cardinality constraints (cont. ) b) One optional, one mandatory A project must be assigned to at least one employee, and may be assigned to many Chapter 3 An employee can be assigned to any number of projects, or may not be assigned to any at all 30
Figure 3 -17 Examples of cardinality constraints (cont. ) c) Optional cardinalities A person is married to at most one other person, or may not be married at all Chapter 3 31
Figure 3 -21 Examples of multiple relationships a) Employees and departments Entities can be related to one another in more than one way Chapter 3 32
Figure 3 -21 Examples of multiple relationships (cont. ) b) Professors and courses (fixed lower limit constraint) Here, min cardinality constraint is 2. At least two professors must be qualified to teach course. Each professor must be qualified to teach at least one course. Chapter 3 33
Figure 3 -15 a and 3 -15 b Multivalued attributes can be represented as relationships simple composite Chapter 3 34
Strong vs. Weak Entities, and Identifying Relationships n Strong entities n n Weak entity n n n exist independently of other types of entities has its own unique identifier underlined with single line dependent on a strong entity (identifying owner)…cannot exist on its own does not have a unique identifier (only a partial identifier) partial identifier underlined with double line entity box has double line Identifying relationship n links strong entities to weak entities Chapter 3 35
Identifying relationship (Figure 3 -5) 扶養親屬 Strong entity Chapter 3 Weak entity 36
Associative Entities entity–has attributes n An n A n When should a relationship–links entities together associative entity n n n with attributes instead be an All relationships for the associative entity should be many The associative entity could have meaning independent of the other entities 有獨立意義時 The associative entity preferably has a unique identifier, and should also have other attributes 自己有獨立id時 The associative entity may participate in other relationships other than the entities of the associated relationship (理由類同下條) Ternary relationships should be converted to associative entities Chapter 3 37
Figure 3 -11 a A binary relationship with an attribute Here, the date completed attribute pertains specifically to the employee’s completion of a course…it is an attribute of the relationship Chapter 3 38
Figure 3 -11 b An associative entity (CERTIFICATE) 轉成一張表 Associative entity is like a relationship with an attribute, but it is also considered to be an entity in its own right Note that the many-to-many cardinality between entities in Figure 3 -11 a has been replaced by two one-to-many relationships with the associative entity Chapter 3 39
Figure 3 -13 c An associative entity – bill of materials structure This could just be a relationship with attributes…it’s a judgment call Chapter 3 40
Figure 3 -18 Ternary relationship as an associative entity Chapter 3 41
Microsoft Visio Notation for Pine Valley Furniture E-R diagram (Figure 3 -22) Different modeling software tools may have different notation for the same constructs 圖例可能略有不同 Chapter 3 42
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