Presentation on theme: "Data Organization & ER Model"— Presentation transcript:
1 Data Organization & ER Model Chapter 2Instructor: Dr. Cynthia Xin ZhangThe slides for this text are organized into several modules. Each lecture contains about enough material for a 1.25 hour class period. (The time estimate is very approximate--it will vary with the instructor, and lectures also differ in length; so use this as a rough guideline.) This covers Lectures 1 and 2 (of 7) in Module (5).Module (1): Introduction (DBMS, Relational Model)Module (2): Storage and File Organizations (Disks, Buffering, Indexes)Module (3): Database Concepts (Relational Queries, DDL/ICs, Views and Security)Module (4): Relational Implementation (Query Evaluation, Optimization)Module (5): Database Design (ER Model, Normalization, Physical Design, Tuning)Module (6): Transaction Processing (Concurrency Control, Recovery)Module (7): Advanced Topics1
2 Data design When we build a new database … Relational database design in DBMSWhen we transform a existing database …Data manipulation (merge, clean, format, etc.)Information Retrieval
3 What Is a DBMS? A very large, integrated collection of data. Models real-world enterprise.Entities (e.g., students, courses)Relationships (e.g., Madonna is taking CSC 132)A Database Management System (DBMS) is a software package designed to maintain and utilize databases.
4 Why Use a DBMS? Data independence and efficient access. Reduced application development time.Data integrity and security.Uniform data administration.Concurrent access, recovery from crashes.3
5 Why study Database implementation? Good job market.Web DeveloperSQL Programmer (development DBA)Database Administrator (production DBA)Data AnalystGraduate schoolDBMS encompasses most of CS
6 Data ModelsA data model is a collection of concepts for describing data.A schema is a description of a particular collection of data, using the a given data model.The relational model of data is the most widely used model today.Main concept: relation, basically a table with rows and columns.Every relation has a schema, which describes the columns, or fields.5
7 Levels of AbstractionMany views, single conceptual (logical) schema and physical schema.Views describe how users see the data.Conceptual schema defines logical structurePhysical schema describes the files and indexes used.View 1View 2View 3Conceptual SchemaPhysical SchemaDDL (CREAT, ALTER, DROP); DML (SELECT, INTERT, UPDATE);DCL (GRANT, REVOKE); TCL (COMMIT, SAVEPOINT, ROLLBACK).6
8 Example: University Database Conceptual schema:Students(sid: string, name: string, login: string, age: integer, gpa:real)Courses(cid: string, cname:string, credits:integer)Enrolled(sid:string, cid:string, grade:string)Physical schema:Relations stored as unordered files.Index on first column of Students.External Schema (View):Course_info(cid:string,enrollment:integer)7
9 Data IndependenceApplications insulated from how data is structured and stored.Logical data independence: Protection from changes in logical structure of data.Physical data independence: Protection from changes in physical structure of data.One of the most important benefits of using a DBMS!
11 Inside a Database Tables Relationship among tables Operations (queries)
12 Overview of db design Security design Requirement analysis Data to be storedApplications to be builtOperations (most frequent) subject to performance requirementConceptual db designDescription of the data (including constraints)By high level model such as ERLogical db designChoose DBMS to implementConvert conceptual db design into database schemaBeyond ER designSchema refinement (normalization)Physical db designAnalyze the workloadIndexingSecurity design
13 Conceptual designIssues to consider: (ER Model is used at this stage.)What are the entities and relationships in the enterprise?What information about these entities and relationships should we store in the database (i.e., attributes)?What are the integrity constraints or business rules that hold?Solution:A database `schema’ in the ER Model can be represented pictorially (ER diagrams).Can map an ER diagram into a relational schema.2
15 EmployeesssnnamelotER Model BasicsEntity: Real-world object distinguishable from other objects. An entity is described (in DB) using a set of attributes.Entity Set: A collection of similar entities. E.g., all employees.All entities in an entity set have the same set of attributes.Each entity set has a key.Each attribute has a domain.What’s should be entities? Attributes?The slides for this text are organized into several modules. Each lecture contains about enough material for a 1.25 hour class period. (The time estimate is very approximate--it will vary with the instructor, and lectures also differ in length; so use this as a rough guideline.) This covers Lectures 1 and 2 (of 6) in Module (5).Module (1): Introduction (DBMS, Relational Model)Module (2): Storage and File Organizations (Disks, Buffering, Indexes)Module (3): Database Concepts (Relational Queries, DDL/ICs, Views and Security)Module (4): Relational Implementation (Query Evaluation, Optimization)Module (5): Database Design (ER Model, Normalization, Physical Design, Tuning)Module (6): Transaction Processing (Concurrency Control, Recovery)Module (7): Advanced TopicsWhat’s the key? How many keys one object can have?3
16 A Universal Data Model for All? NamessnLocationBudgetEmployeesDepartmentsCompaniesNameBusiness
17 KeyA key is a minimal set of attributes whose values uniquely identify an entity in the set.Candidate key.Primary key.
18 Entity, Entity Set, Attribute, and Schema ID or SSNNameUserIDAgeGPAJohn Smithjsmith213.68Miki Jordanmjordan283.45David Kimdkim254.00Paul Lee26plee3.89
19 ER Model Basics (Contd.) sincenamednamessnlotdidbudgetEmployeesWorks_InDepartmentsRelationship: Association among 2 or more entities. E.g., Sam works in the Accounting Department.Relationship Set: Collection of similar relationships. E.g., Many individuals works in many different departments.4
20 Entity vs. Entity SetExample: StudentJohn Smith( , John Smith, 18, 3.5)Students in CSC439, John Smith, 18, 3.5, Jie Zhang, 20, 3.0, Anil Jain, 21, 3.8
21 Example of KeysPrimary keyCandidate key, John Smith, 18, 3.5, Jie Zhang, 20, 3.0, Anil Jain, 21, 3.8
22 Relationship vs. Relationship Set John Smith( , John Smith, 18, 3.5)RelationshipITCS3160(3160, ITCS, DBMS, J. Fan, 3, Kenn. 236)
23 Relationship vs. Relationship Set , John Smith, 18, 3.5Students, Jie Zhang, 20, 3.0, Anil Jain, 21, 3.8Relationship set(“Enrolled in”)3160, ITCS, DBMS, J. Fan, 3, Kenn. 236Courses6157, ITCS, Visual DB, J. Fan, 3, Kenn. 236
24 Relationship vs. Relationship Set LoginAgeNameCreditNameIdRoomGPAIdStudentsCoursesEnrolled_InDescriptive attributeGrade
25 Example 1 Build an ER-diagram for a university database: Students Have an Id, Name, Login, Age, GPACoursesHave an Id, Name, Credit HoursStudents enroll in coursesReceive a grade2
26 Example 2 Build an ER Diagram for a hospital database: Patients Name, Address, Phone #, AgeDrugsName, Manufacturer , Expiration DatePatients are prescribed of drugsDosage, # Days2
29 Mary studies in the CS Dept. Tom studies in the CS Dept. Potential Relationship TypesStudentsCS DeptIN?Mary studies in the CS Dept.Tom studies in the CS Dept.Jack studies in the CS Dept.…The CS Dept has lots of students.No student in the CS Dept works in other else Dept at the same time.
30 Mary is taking the ITCS3160,ITCS2212. Potential Relationship Types??StudentstakeCoursesMary is taking the ITCS3160,ITCS2212.Tom is taking the ITCS3160, ITCS2214.Jack is taking the ITCS1102, ITCS2214.…61 students are taking ITCS3160.120 students are taking ITCS2214.
31 Key Constraints Consider Works_In: An employee can work in many departments;A dept can have many employees.sincenamednamessnlotdidbudgetEmployeesWorks_InDepartments
32 Key Constraints Consider Works_In: An employee can work in at most one department;A dept can have many employees.sincenamednamessnlotdidbudgetEmployeesWorks_InDepartmentsWhy "work-in" is not "key constraint"??
33 Key Constraints At most one!!! sinceIn contrast, each dept has at most one manager, according to the key constraint on Manages.lotnamessndnamedidbudgetManagesEmployeesDepartmentsKey Constraint(time constraint)6
34 Participation Constraints Does every department have a manager?If so, this is a participation constraint: the participation of Departments in Manages is said to be total (vs. partial).Every did value in Departments table must appear in a row of the Manages table (with a non-null ssn value!)sincesincenamenamednamednamessnlotdiddidbudgetbudgetEmployeesManagesDepartmentsPartialTotal w/keyconstraintTotalWorks_InTotalsince8
35 What are the policies behind this ER model? sincesincenamenamednamednamessnlotdiddidbudgetbudgetEmployeesManagesDepartmentsTotalTotal & keyconstraintTotalWorks_InTotalsince
36 Any Difference? Total w/key Partial constraint Total Total since lot namessndnamedidbudgetManagesEmployeesDepartmentsAny Difference?Works_InsincesincenamenamednamednamessnlotdiddidbudgetbudgetEmployeesManagesDepartmentsPartialTotal w/keyconstraintTotalWorks_InTotalsince8
37 Weak Entities vs. Owner Entities A weak entity can be identified uniquely only by considering the primary key of another (owner) entity.Owner entity set and weak entity set must participate in a one-to-many relationship set (1 owner, many weak entities).Weak entity set must have total participation in this identifying relationship set.namecostssnpnamelotagePrimary Keyfor weak entityEmployeesPolicyDependentsIdentifying RelationshipWeak Entity10
38 Ternary Relationship Why? name dname budget did ssn lot Works_In3 DepartmentsEmployeesDurationfromtoWhy?sincenamednamessnlotdidbudgetEmployeesWorks_InDepartments
39 ISA (`is a’) Hierarchies nameISA (`is a’) HierarchiesssnlotEmployeesAs in C++, or other PLs, attributes are inherited.hourly_wageshours_workedISAcontractidIf we declare A ISA B, every A entity is also considered to be a B entity.Hourly_EmpsContract_EmpsOverlap constraints: Can Joe be an Hourly_Emps as well as a Contract_Emps entity? (Allowed/disallowed)Covering constraints: Does every Employees entity also have to be an Hourly_Emps or a Contract_Emps entity? (Yes/no)Reasons for using ISA:To add descriptive attributes specific to a subclass.To identify entitities that participate in a relationship.12
40 nameAggregationssnlotEmployeesUsed when we have to model a relationship involving (entitity sets and) a relationship set.Aggregation allows us to treat a relationship set as an entity set for purposes of participation in (other) relationships.Monitors mapped to table like any other relationship set.MonitorsuntilAggregationstarted_ondnamepidpbudgetdidbudgetProjectsSponsorsDepartments2
41 Real Database DesignBuild an ER Diagram for the following information:Walmart StoresStore Id, Address, Phone #ProductsProduct Id, Description, PriceManufacturersName, Address, Phone #Walmart Stores carry productsAmount in storeManufacturers make productsAmount in factory/warehouses2
42 Conceptual Design Using the ER Model Design choices:Should a concept be modeled as an entity or an attribute?Should a concept be modeled as an entity or a relationship?Identifying relationships: Binary or Ternary? Aggregation?Always follow the requirements.3
43 Entity vs. AttributeShould address be an attribute of Employees or an entity (connected to Employees by a relationship)?Depends upon the use we want to make of address information, and the semantics of the data:If we have several addresses per employee, address must be an entity (since attributes cannot be set-valued).If the structure (city, street, etc.) is important, e.g., we want to retrieve employees in a given city, address must be modeled as an entity (since attribute values are atomic).
44 Entity vs. Attribute (Contd.) fromtonamednameWorks_In2 does not allow an employee to work in a department for two or more periods.Similar to the problem of wanting to record several addresses for an employee: we want to record several values of the descriptive attributes for each instance of this relationship.ssnlotdidbudgetEmployeesWorks_In2DepartmentsnamednamebudgetdidssnlotWorks_In3EmployeesDepartmentsDurationfromto5
45 Entity vs. Relationship First ER diagram OK if a manager gets a separate discretionary budget for each dept.Redundancy of dbudget, which is stored for each dept managed by the manager.Misleading: suggests dbudget tied to managed dept.What if a manager gets a discretionary budget that covers all managed depts?sincedbudgetnamednamessnlotdidbudgetEmployeesManages2DepartmentsnamednamessnlotdidbudgetEmployeesManages3DepartmentssinceIsAManagerdbudget6
46 Binary vs. Ternary Relationships *nameEmployeesssnlotpnameageCoversDependentsBad designPoliciespolicyidcostRequirements:A policy not to be owned by more than one employee.Every policy must be owned by some employee.Dependents is a weak entity set. Each identified by pname +policyid7
47 Binary vs. Ternary Relationships *nameEmployeesssnlotpnameageIf each policy is owned by just 1 employee:Key constraint on Policies would mean policy can only cover 1 dependent!CoversDependentsBad designPoliciespolicyidcostBeneficiaryagepnameDependentspolicyidcostPoliciesPurchasernameEmployeesssnlotBetter design
48 Binary vs. Ternary Relationships (Contd.) Previous example illustrated a case when binary relationships were better than one ternary relationship.An example in the other direction: a ternary relation Teaches relates entity set Students, Professors and Courses, and has descriptive attributes term and year. No combination of binary relationships is an adequate substitute:P teaches S, S takes C, P teaches C, do not necessarily imply that P indeed teaches S of C!How do we record term and year?8
50 Summary of Conceptual Design Conceptual design follows requirements analysis,Yields a high-level description of data to be storedER model popular for conceptual designConstructs are expressive, close to the way people think about their applications.Basic constructs: entities, relationships, and attributes (of entities and relationships).Some additional constructs: weak entities, ISA hierarchies, and aggregation.Note: There are many variations on ER model.11
51 Summary of ER (Contd.)Several kinds of integrity constraints can be expressed in the ER model: key constraints, participation constraints, and overlap/covering constraints for ISA hierarchies. Some foreign key constraints are also implicit in the definition of a relationship set.Some constraints (notably, functional dependencies) cannot be expressed in the ER model.Constraints play an important role in determining the best database design for an enterprise.12
52 Summary of ER (Contd.)ER design is subjective. There are often many ways to model a given scenario! Analyzing alternatives can be tricky, especially for a large enterprise. Common choices include:Entity vs. attribute, entity vs. relationship, binary or n-ary relationship, whether or not to use ISA hierarchies, and whether or not to use aggregation.Ensuring good database design: resulting relational schema should be analyzed and refined further. FD information and normalization techniques are especially useful.13
53 Example 1 Answer Login Age Name Credit Name Id GPA Id Students Courses Enrolled_InGrade2
54 Example 2 Answer Addr Phone Manuf Exp Name Name Age Patients Drugs Prescribed#daysDosage2