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Database Systems DataBase System Major Content & Grade Introduction * The *** Relational Model SQL **** Transaction Management *** Database Design (E-R) *** Database Design (Normalization) *** Haichang Gao , Software School , Xidian University 2 DataBase System Ch06 Database design Part 1 Conceptual Design Part 2 Logical Design Haichang Gao , Software School , Xidian University 3 DataBase System Contents Part 1 Conceptual Design Introduction Modeling Constraints E-R Diagram Design Issues Weak Entity Sets Extended E-R Features Design of the Bank Database Reduction to Relation Schemas Database Design UML Haichang Gao , Software School , Xidian University 4 DataBase System Introduction DataBase Design is a subject of research ever since the late 1970s Database system typically have only a very limited understanding of what the data in the database means. Semantic model(语义模型) Extended model managed to capture all of the semantics of the situation under consideration Entity-relationship approach Most commonly used in practice Haichang Gao , Software School , Xidian University 5 DataBase System Introduction Entity-Relationship model is a set of concepts and graphical symbols that can be used to create conceptual schemas. Versions model — Peter Chen (1976). Extended E-R model — Extensions to the Chen model. Information Engineering (IE) — James Martin (1990); it uses “crow’s foot” notation, is easier to understand. IDEF1X — A national standard developed by the National Institute of Standards and Technology(NIST) . Unified Modeling Language (UML) — The Object Management Group; it supports object-oriented methodology. Original E-R Haichang Gao , Software School , Xidian University 6 DataBase System Modeling A database can be modeled as: A collection of entities(实体) Relationship(联系) among entities. An entity is an object that exists and is distinguishable from other objects. Example: specific person, company, event, plant Entities have attributes(属性) Example: people have names and addresses Haichang Gao , Software School , Xidian University 7 DataBase System Entity Sets An entity set (entity class, 实体集) is a set of entities of the same type that share the same properties. Example: set of all persons, companies, trees, holidays customer_id customer_ customer_ customer_ name street city loan_ amount number Haichang Gao , Software School , Xidian University 8 DataBase System Entity Sets An entity is a collection of distinguishable realworld objects with common properties. A value of an entity is known as entity instance(实例), or sometimes an entity occurrence(事件) Any entity instance can be represented and distinguished only by its own properties Haichang Gao , Software School , Xidian University 9 DataBase System Attributes Domain(域) – the set of permitted values for each attribute Attribute types: Simple and composite attributes(单一属性和组合属性). Single-valued attributes (单值属性) and multi-valued attributes(多值属性) E.g. Derived multivalued attribute: phone-numbers attributes(派生属性) Can be computed from other attributes E.g. age, given date of birth Haichang Gao , Software School , Xidian University 10 DataBase System Relationship Sets A relationship(联系) is an association among several entities. an ordered list of m entities, E1,E2,…,Em ( where the same entity may occur more than once in the list), A relationship set is a mathematical relation among n 2 entities, each taken from entity sets : Given {(e1, e2, … en) | e1 E1, e2 E2, …, en En} where (e1, e2, …, en) is a relationship Haichang Gao , Software School , Xidian University 11 DataBase System Relationship Sets Haichang Gao , Software School , Xidian University 12 DataBase System Relationship Sets An attribute can also be property of a relationship set. For instance, the depositor relationship set between entity sets customer and account may have the attribute accessdate Haichang Gao , Software School , Xidian University 13 DataBase System Relationship Sets Degree of a Relationship Set Refers to number of entity sets that participate in a relationship set. Relationship sets that involve two entity sets are binary (or degree two). Generally, most relationship sets are binary. Relationship sets may involve more than two entity sets. E.g. Suppose employees of a bank may have jobs (responsibilities) at multiple branches, with different jobs at different branches. Then there is a ternary relationship set between entity sets employee, job and branch. Relationships between more than two entity sets are rare. Haichang Gao , Software School , Xidian University 14 DataBase System Mapping Cardinalities Express the number of entities to which another entity can be associated via a relationship set. Most useful in describing binary relationship sets. For a binary relationship set the mapping cardinality must be one of the following types: One to one(1:1) One to many (1:M) Many to one (M:1) Many to many (M:N) Haichang Gao , Software School , Xidian University 15 DataBase System Mapping Cardinalities One to one One to many Note: Some elements in A and B may not be mapped to any elements in the other set Haichang Gao , Software School , Xidian University 16 DataBase System Mapping Cardinalities Many to one Many to many Note: Some elements in A and B may not be mapped to any elements in the other set Haichang Gao , Software School , Xidian University 17 DataBase System Mapping Cardinalities Can make access-date an attribute of account, instead of a relationship attribute, if each account can have only one customer. Haichang Gao , Software School , Xidian University 18 DataBase System Keys for Entity Sets A super key of an entity set is a set of one or more attributes whose values uniquely determine each entity. A candidate key of an entity set is a minimal super key Customer_id is candidate key of customer account_number is candidate key of account Although several candidate keys may exist, one of the candidate keys is selected to be the primary key. Haichang Gao , Software School , Xidian University 19 DataBase System Keys for Relationship Sets The combination of primary keys of the participating entity sets forms a super key of a relationship set. (customer_id, account_number) is the super key of depositor NOTE: this means a pair of entity sets can have at most one relationship in a particular relationship set. Must consider the mapping cardinality of the relationship set when deciding what are the candidate keys Need to consider semantics of relationship set in selecting the primary key in case of more than one candidate key Haichang Gao , Software School , Xidian University 20 DataBase System E-R Diagrams Rectangles represent entity sets. Diamonds represent relationship sets. Lines link attributes to entity sets and entity sets to relationship sets. Ellipses represent attributes Double ellipses represent multivalued attributes. Dashed ellipses denote derived attributes. Underline indicates primary key attributes Haichang Gao , Software School , Xidian University 21 DataBase System E-R Diagrams E-R Diagram With Composite, Multivalued, and Derived Attributes Haichang Gao , Software School , Xidian University 22 DataBase System E-R Diagrams Relationship Sets with Attributes Haichang Gao , Software School , Xidian University 23 DataBase System Roles(角色) Entity sets of a relationship need not be distinct labels “manager” and “worker” are called roles; they specify how employee entities interact via the works-for relationship set. The Haichang Gao , Software School , Xidian University 24 DataBase System Roles(角色) Roles are indicated in E-R diagrams by labeling the lines that connect diamonds to rectangles. Role labels are optional, and are used to clarify semantics of the relationship Haichang Gao , Software School , Xidian University 25 DataBase System Cardinality Constraints We express cardinality constraints by drawing either a directed line (), signifying “one,” or an undirected line (—), signifying “many,” between the relationship set and the entity set. Haichang Gao , Software School , Xidian University 26 DataBase System One-To-One Relationship In the one-to-one relationship A customer is associated with at most one (including 0) loan via the relationship borrower, and A loan is associated with at most one (including 0) customer via borrower . Haichang Gao , Software School , Xidian University 27 DataBase System One-To-Many Relationship In the one-to-many relationship A customer is associated with several (including 0) loans via borrower; A loan is associated with at most one customer via borrower. Haichang Gao , Software School , Xidian University 28 DataBase System Many-To-One Relationships In a many-to-one relationship A customer is associated with at most one loan via borrower; A loan is associated with several (including 0) customers via borrower. Haichang Gao , Software School , Xidian University 29 DataBase System Many-To-Many Relationship In a many-to-many relationship A customer is associated with several (possibly 0) loans via borrower; A loan is associated with several (possibly 0) customers via borrower . Haichang Gao , Software School , Xidian University 30 DataBase System Ternary Relationship Suppose employees of a bank may have jobs (responsibilities) at multiple branches, with different jobs at different branches. Then there is a ternary relationship set between entity sets employee, job and branch. Haichang Gao , Software School , Xidian University 31 DataBase System Cardinality of Entity Participation Total participation (indicated by double line): Every entity in the entity set participates in at least one relationship in the relationship set. E.g. participation of loan in borrower is total, for EACH loan must have a customer associated to it via borrower Partial participation: Some entities may not participate in any relationship in the relationship set. E.g. participation of customer in borrower is partial Haichang Gao , Software School , Xidian University 32 DataBase System Cardinality of Entity Participation When an entity E takes part in a relationship R with min- card(E,R)=x (x is either 0 or 1) and max-card(E,R)=y (y is either 1 or N), then in the E-R diagram the connecting line between E and R can be labeled with the ordered cardinality pair (x,y). We use a new notation to represent this minimum-maximum pair (x,y): card(E,R) = (x,y). Haichang Gao , Software School , Xidian University 33 DataBase System Cardinality of Entity Participation Cardinality limits can also express participation constraints E.g. participation of loan in borrower is total, for EACH loan must have a customer associated to it via borrower ; participation of customer in borrower is partial The relationship borrower is one-to-many from customer to loan . Haichang Gao , Software School , Xidian University 34 DataBase System Design Issues Use of entity sets vs. attributes Choice mainly depends on the structure of the enterprise being modeled, and on the semantics associated with the attribute in question. 属性间不产生联系,当属性与其他实体有联系时,该属 性应作为实体。 职工 职工号 姓名 职工 职称 年龄 职工号 聘任 年龄 姓名 职称 职称名 工资 福利 住房 Haichang Gao , Software School , Xidian University 35 DataBase System Design Issues Use of entity sets vs. attributes 当一个对象被表达为实体时,在其它实体中不能以属 性出现,只能用联系表达。 病人 住院号 姓名 病房 病房 病房号 位置 年龄 病人 住院号 姓名 年龄 住院 房位数 面积 病房 病房号 位置 房位数 面积 Haichang Gao , Software School , Xidian University 36 DataBase System Design Issues Use of entity sets vs. relationship sets Possible guideline is to designate a relationship set to describe an action that occurs between entities 供应商 订单 供应商 供应量 供应 项目 订单 零件 项目 供应量 零件 Haichang Gao , Software School , Xidian University 37 DataBase System Design Issues Binary vs. n-ary relationship sets Although it is possible to replace any nonbinary (n-ary, for n > 2) relationship set by a number of distinct binary relationship sets, a n-ary relationship set shows more clearly that several entities participate in a single relationship. 姓名 姓名 编号 学历 教师 姓名 学号 性别 年龄 编号 学历 教师 姓名 学号 学生 学生 代课 学习 学习 成绩 学分 课程名 成绩 课程 课程 课程号 性别 年龄 学分 课程号 课程名 Haichang Gao , Software School , Xidian University 38 DataBase System Design Issues Placement of relationship attributes customer_street customer_name account_number access_date customer_city customer_id one-to-many customer 0..N customer_name depositor 1..1 account customer_street account_number customer_city customer_id customer 0..N depositor 1..N account many-to-many access_date Haichang Gao , Software School , Xidian University 39 Database Systems DataBase System Contents Part 1 Conceptual Design Introduction Modeling Constraints E-R Diagram Design Issues Weak Entity Sets Extended E-R Features Design of the Bank Database Reduction to Relation Schemas Database Design UML Haichang Gao , Software School , Xidian University 41 DataBase System Weak Entity Sets An entity set that does not have a primary key is referred to as a weak entity set (弱实体).. The existence of a weak entity set depends on the existence of a identifying entity set (strong entity, 标识实体,强实体). It must relate to the identifying entity set via a total, one-tomany relationship set from the identifying to the weak entity set Haichang Gao , Software School , Xidian University 42 DataBase System Weak Entity Sets The discriminator (or partial key) of a weak entity set is the set of attributes that distinguishes among all the entities of a weak entity set. The primary key of a weak entity set is formed by the primary key of the strong entity set on which the weak entity set is existence dependent, plus the weak entity set’s discriminator. Haichang Gao , Software School , Xidian University 43 DataBase System Weak Entity Sets We depict a weak entity set by double rectangles. Identifying relationship depicted using a double diamond. We underline the discriminator of a weak entity set with a dashed line. Example: Primary key for payment : (loan_number, payment_number) Haichang Gao , Software School , Xidian University 44 DataBase System Weak Entity Sets In a university, a course is a strong entity and a course- offering can be modeled as a weak entity The discriminator of course-offering would be semester (including year) and section-number (if there is more than one section) C# Cname Cridet course semester offering sectionnumber classes Courseoffering Haichang Gao , Software School , Xidian University 45 DataBase System Weak Entity Sets In a university, a course is a strong entity and a course- offering can be modeled as a weak entity The discriminator of course-offering would be semester (including year) and section-number (if there is more than one section) If we model course-offering as a strong entity we would model course-number as an attribute. Then the relationship with course would be implicit in the course-number attribute sectionC# Cname Cridet course semester number classes C# offering Courseoffering Haichang Gao , Software School , Xidian University 46 DataBase System Contents Extended E-R Features Specialization Generalization Aggregation Summary of Symbols Haichang Gao , Software School , Xidian University 47 DataBase System Specialization(特殊化) Top-down design process - we designate subgroupings within an entity set that are distinctive from other entities in the set. These subgroupings become lower-level entity sets that have attributes or participate in relationships that do not apply to the higher-level entity set. Depicted by a triangle component labeled ISA (E.g. customer “is a” person). person ISA customer Haichang Gao , Software School , Xidian University 48 DataBase System Specialization(特殊化) Attribute inheritance(属性继承) – a lower-level entity set inherits all the attributes and relationship participation of the higher-level entity set to which it is linked. Example of entity type hierarchy (not E-R Diagram) Haichang Gao , Software School , Xidian University 49 DataBase System Specialization Example Haichang Gao , Software School , Xidian University 50 DataBase System Generalization(一般化,泛化) A bottom-up(自底向上) design process– combine a number of entity sets that share the same features into a higher-level entity set. Specialization and generalization are simple inversions of each other; they are represented in an E-R diagram in the same way. The terms specialization and generalization are used interchangeably. Haichang Gao , Software School , Xidian University 51 DataBase System Specialization and Generalization Can have multiple specializations of an entity set based on different features. E.g. permanent-employee vs. temporary-employee, in addition to officer vs. secretary vs. teller Haichang Gao , Software School , Xidian University 52 DataBase System Specialization and Generalization Each particular employee would be a member of one of permanent-employee or temporaryemployee, and also a member of one of officer, secretary, or teller The ISA relationship also referred to as superclass subclass relationship Haichang Gao , Software School , Xidian University 53 DataBase System Aggregation Relationship sets works_on and manages represent overlapping information Every manages relationship corresponds to a works_on relationship However, some works_on relationships may not correspond to any manages relationships Haichang Gao , Software School , Xidian University 54 DataBase System Aggregation Eliminate this redundancy via aggregation: Treat relationship as an abstract entity Allows relationships between relationships Abstraction of relationship into new entity Haichang Gao , Software School , Xidian University 55 DataBase System Aggregation Without introducing redundancy, the following diagram represents: An employee works on a particular job at a particular branch An employee, branch, job combination may have an associated manager Haichang Gao , Software School , Xidian University 56 DataBase System E-R Design Decisions The use of an attribute or entity set to represent an object. Whether a real-world concept is best expressed by an entity set or a relationship set. The use of a ternary relationship versus a pair of binary relationships. The use of a strong or weak entity set. The use of specialization/generalization – contributes to modularity in the design. The use of aggregation – can treat the aggregate entity set as a single unit without concern for the details of its internal structure. Haichang Gao , Software School , Xidian University 57 DataBase System Haichang Gao , Software School , Xidian University 58 DataBase System Summary of Symbols Used in ERD Haichang Gao , Software School , Xidian University 59 DataBase System Summary of Symbols -Cont. Haichang Gao , Software School , Xidian University 60 DataBase System Alternative E-R Notations Haichang Gao , Software School , Xidian University 61 DataBase System Contents UML Haichang Gao , Software School , Xidian University 62 DataBase System UML UML: Unified Modeling Language UML has many components to graphically model different aspects of an entire software system UML Class Diagrams correspond to E-R Diagram, but several differences. Haichang Gao , Software School , Xidian University 63 DataBase System UML Haichang Gao , Software School , Xidian University 64 DataBase System UML Entity sets are shown as boxes, and attributes are shown within the box, rather than as separate ellipses in E-R diagrams. Binary relationship sets are represented in UML by just drawing a line connecting the entity sets. The relationship set name is written adjacent to the line. The role played by an entity set in a relationship set may also be specified by writing the role name on the line, adjacent to the entity set. The relationship set name may alternatively be written in a box, along with attributes of the relationship set, and the box is connected, using a dotted line, to the line depicting the relationship set. Non-binary relationships drawn using diamonds, just as in ER diagrams Haichang Gao , Software School , Xidian University 65 DataBase System UML * Note reversal of position in cardinality constraint depiction * Generalization can use merged or separate arrows independent of disjoint/overlapping Haichang Gao , Software School , Xidian University 66 DataBase System UML Cardinality constraints are specified in the form l..h, where l denotes the minimum and h the maximum number of relationships an entity can participate in. Beware: the positioning of the constraints is exactly the reverse of the positioning of constraints in E-R diagrams. The constraint 0..* on the E2 side and 0..1 on the E1 side means that each E2 entity can participate in at most one relationship, whereas each E1 entity can participate in many relationships; in other words, the relationship is many to one from E2 to E1. Single values, such as 1 or * may be written on edges; The single value 1 on an edge is treated as equivalent to 1..1, while * is equivalent to 0..*. Haichang Gao , Software School , Xidian University 67 DataBase System Mapping E-R to Relation Schemas Primary keys allow entity sets and relationship sets to be expressed uniformly as relation schemas that represent the contents of the database. A database which conforms to an E-R diagram can be represented by a collection of schemas. For each entity set and relationship set there is a unique schema that is assigned the name of the corresponding entity set or relationship set. Each schema has a number of columns (generally corresponding to attributes), which have unique names Haichang Gao , Software School , Xidian University 68 DataBase System Representing Entity Sets as Schemas Each entity in an E-R diagram is mapped to a single table in a relational database. The columns of the table represent all the single-valued attributes that are attached to the entity; Composite attributes are flattened out by creating a separate attribute for each component attribute; A multivalued attribute M of an entity E is represented by a separate schema EM Derived attributes are not represent in the relational shema. Haichang Gao , Software School , Xidian University 69 DataBase System Representing Entity Sets as Schemas Relational Schema customer( customer-id, first-name, middle-initial, last-name, street-number, street-name, apartment-number, city,state, zipdoce, date-of-birth ) customer-phone( customer-id, phone-number) Haichang Gao , Software School , Xidian University 70 DataBase System Representing Entity Sets as Schemas A strong entity set reduces to a schema with the same attributes. A weak entity set becomes a table that includes a column for the primary key of the identifying strong entity set Relational Schema loan(loan_number, amount) payment(loan_number, payment_number, payment_date, payment_amount ) 注:依赖联系已体现! Haichang Gao , Software School , Xidian University 71 DataBase System Representing Relationship Sets as Schemas A many-to-many relationship set is represented as a schema with attributes for the primary keys of the two participating entity sets, and any descriptive attributes of the relationship set. Relational Schema customer(…), loan(…) borrower (customer_id, loan_number ) Haichang Gao , Software School , Xidian University 72 DataBase System Representing Relationship Sets as Schemas Many-to-one and one-to-many relationship sets that are total on the many-side can be represented by adding an extra attribute to the “many” side, containing the primary key of the “one” side. Relational Schema branch (branch-name, branch-city, assets) account (account-number, balance, branch-name) Haichang Gao , Software School , Xidian University 73 DataBase System Representing Relationship Sets as Schemas For one-to-one relationship sets, either side can be chosen to act as the “many” side If participation is partial on the “many” side, replacing a schema by an extra attribute in the schema corresponding to the “many” side could result in null values. Relational Schema …… Haichang Gao , Software School , Xidian University 74 DataBase System Representing Specialization via Schemas Haichang Gao , Software School , Xidian University 75 DataBase System Representing Specialization via Schemas Method 1: Form a schema for the higher-level entity Form a schema for each lower-level entity set, include primary key of higher-level entity set and local attributes Haichang Gao , Software School , Xidian University 76 DataBase System Representing Specialization via Schemas Method 1: Relational Schema person( name, street, city ) customer( name, credit_rating ) employee( name, salary ) ...... Haichang Gao , Software School , Xidian University 77 DataBase System Representing Specialization via Schemas Method 2: Form a schema for each entity set with all local and inherited attributes Haichang Gao , Software School , Xidian University 78 DataBase System Representing Specialization via Schemas Method 2: Relational Schema person( name, street, city ) customer( name, street, city, credit_rating ) employee( name, street, city, salary ) ...... Haichang Gao , Software School , Xidian University 79 DataBase System Schemas Corresponding to Aggregation To represent aggregation, create a schema containing primary key of the aggregated relationship, the primary key of the associated entity set any descriptive attributes Haichang Gao , Software School , Xidian University 80 DataBase System Schemas Corresponding to Aggregation Relational Schema ......(略去相关实体转换的关系模式) manages (employee_id, branch_name, title, manager_name) NOTE: Schema works_on is redundant provided, we willing to store null values for attribute manager_name in manages. Haichang Gao , Software School , Xidian University 81 DataBase System Design of the Bank Database Haichang Gao , Software School , Xidian University 82 DataBase System 作业 某医院为了规范人员、财务、药品等的日常管理,决定采用数据 库技术实现医院信息系统。现负责人提出要求如下: 1. 医院有若干个科室,科室信息有科室编号,科室名称,楼 层等; 2. 医院药品信息有药品名称,厂商,售价等; 3. 医生有统一的编号、姓名、性别、职称等信息,并属于某 一个科室,门诊时间由医院统一安排; 4. 病人看病时选择科室、医生进行挂号,医生根据病情开具 处方,处方具体包含病人姓名,年龄,性别,日期,药品 清单。 根据以上需求,完成该医院信息系统的设计及编程(要求使用上 文中的名称,如需引入新名称,必需使用与实际意义相符的中文 名称): a) 画出系统的完整E-R图; b) 将你的E-R图转换为关系模式 P256 6.2 , 6.21 Haichang Gao , Software School , Xidian University 83