EECE 433 Chapter 2, Spring 2012

Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke The Entity-Relationship Model Chapter 2 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke Motivating Example: Design DB for B&N Hiring Dudes DB Co.  Requirements:  Topic: Books focusing on horse racing  Customers to browse catalog over the internet  Order books over the internet  Order contains: ISBN number, and quantity.  Shipment of whole order in one shot. If not all books in stock, then additional copies made.  For each book, catalog contains: ISBN, title, author, purchase price, sales price, and year book was published.  Records for customers are maintained: names and addresses.  New customers have to open an account first, and get CID.  Payment by credit card HOW??? Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke Outline  Database Design and ER Diagram  Entities, Attributes, & Entity set  Relationships and relationship Set  Additional Features of the ER Model  Conceptual design with the ER Model  Conceptual Design for Large Enterprises  Case Study: Internet Shop Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke Database Design Approach 1. Requirement Collection 2. Requirement Analysis 3. Conceptual Design (Ch. 2) 4. Logical Design (Ch. 3) 5. Physical Implementation (Ch. 8) 6. Tuning & possibly repeating above steps (Ch. 4, 5, 19) Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke Requirements Analysis  Understand (in english) what is being asked:  What data to store in database?  What application(s) must be built on top of it?  What operations are most frequent and corresponding performance requirements?  Expected changes on current environment?  Note that this step may be an informal process of discussion with the customer(s) Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke Conceptual Design  In this step, develop a high level description of the data to be stored in DB and their constraints.  Conceptual design: (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?  What are the integrity constraints or business rules that hold?  Represent the database `schema’ in the ER Model pictorially (ER diagrams).  Then, map the ER diagram into a relational schema. Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke Remaining steps in the design  Logical Design  Translate ER Diagram to Relations (Ch. 3)  Physical Implementation  E.g. Choosing Indexing (Ch. 8)  Tuning & possibly repeating above steps  E.g. Normalization (Ch. 19)  …Other considerations, e.g. applications (Ch.6-7), security, … Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke Outline  Database Design and ER Diagram  Entities, Attributes, & Entity set  Relationships and relationship Set  Additional Features of the ER Model  Conceptual design with the ER Model  Conceptual Design for Large Enterprises  Case Study: Internet Shop Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke Entity-Relationship Data Model (ER)  entity: a real-world object such as a student.  attributes: to describe a particular entity (e.g. name, SS#, height).  domain: Each attribute comes from a specified domain (e.g., name may be a 20 character string; SS# is a nine-digit integer)  entity set: a collection of similar entities (i.e., those which are distinguished using the same set of attributes. E.g. Football Players  key: a minimal set of attributes for an entity set. May be a single attribute (such as SS#) or multiple attributes as a key ("Bob from Accounting"). We will generally designate one such key as the primary key.  Relationships between entities: Association between entities Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 1 ER Diagram - Entities  An entity set is drawn as a rectangle.  Attributes are drawn as ovals.  Attributes which belong to the primary key are underlined. Employees ssn name lot Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 1 Outline  Database Design and ER Diagram  Entities, Attributes, & Entity set  Relationships and relationship Set  Additional Features of the ER Model  Conceptual design with the ER Model  Conceptual Design for Large Enterprises  Case Study: Internet Shop Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 1 Relationships  Relationship: Association among two or more entities. E.g., Mia works in the Pharmacy department. Ahmad reports to Mia.  Relationship Set: Collection of similar relationships.  An n-ary relationship set R relates n entity sets E1 ... En; each relationship in R involves entities e1 є E1, ..., en є En. Example above: E1, E2 == Employees, Departments. Different types of employee entities working in different department entities.  Same entity set could participate in different relationship sets, or in different “roles” in same set. Example: Reports_To lot dname budget did since name Works_In Departments Employees ssn Reports_To lot name Employees subor- dinate super- visor ssn Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 1 ER Model Basics – Relationships (cont-d)  Descriptive attribute for relationship to record information about the relationship.  An instance of a relationship: A “snapshot” in time of the relationship set. since Works_In 123 456 789 Employees Works-in Feb 08 Jan 09 1 2 3 Departments Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 1 Types of relationships  Consider Works_In: An employee can work in many departments; a dept can have many employees. Many-to-Many 1-to-1 1-to Many Many-to-1 dname budget did since lot name ssn Works_in Employees Departments One employee, one dept One employee, works_in many departments One employee works at most in one dept Different dept can have different employees Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 1 Outline  Database Design and ER Diagram  Entities, Attributes, & Entity set  Relationships and relationship Set  Additional Features of the ER Model  Conceptual design with the ER Model  Case Study: Internet Shop Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 1 Key Constraint  Assume each dept has at most one manager, according to the key constraint on Manages.  Given a department, we can uniquely identify the Manages relationship in which it appears  Key constraint represents a many (depts) to 1 (employee)  Represented by an arrow dname budget did since lot name ssn Manages Employees Departments Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 1 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). • Represented by “bold lines” • Every did value in Departments table must appear in a row of the Manages table (with a non-null ssn value!) lot name dname budget did since name dname budget did since Manages since Departments Employees ssn Works_In Also every employee must be in a dept Also every dept must have employees Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 1 Dependent “pname” is not unique, unless associated with employee sid Weak 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 (one owner, many weak entities).  Weak entity set must have total participation in this identifying relationship set.  Represented by bold boxes around them. Their key is dashed. lot name age pname Dependents Employees ssn Policy cost Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 1 Question  How do we represent many to 1, 1 to 1, and many to many? budget did pid started_on pbudget dname Departments Projects Sponsors since Stopped here in MWF section Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 2 ISA (`is a’) Hierarchies Contract_Emps name ssn Employees lot hourly_wages ISA Hourly_Emps contractid hours_worked • As in C++, or other PLs, attributes are inherited. • If we declare A ISA B, every A entity is also considered to be a B entity.  Reasons for using ISA:  To add descriptive attributes specific to a subclass.  To identify (specific sub-class) entitities that participate in a relationship. Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 2 Aggregation  Used when we have to model a relationship set involved in another relationship set.  Aggregation allows us to treat a relationship set as an entity set for purposes of participation in (other) relationships.  Represented by dashed box.  Aggregation vs. ternary relationship: • Monitors is a distinct relationship, with a descriptive attribute. • Also, can say that each sponsorship is monitored by at most one employee. budget did pid started_on pbudget dname until Departments Projects Sponsors Employees Monitors lot name ssn since Other good examples: OGC vs RPD, prog mgrs vs people mgrs Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 2 Outline  Database Design and ER Diagram  Entities, Attributes, & Entity set  Relationships and relationship Set  Additional Features of the ER Model  Conceptual design with the ER Model  Case Study: Internet Shop Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 2 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?  Constraints in the ER Model:  A lot of data semantics can (and should) be captured.  But some constraints cannot be captured in ER diagrams. Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 2 Entity vs. Attribute  Should 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). Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 2 Entity vs. Attribute (Contd.)  Works_In4 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.  Accomplished by introducing new entity set, Duration. name Employees ssn lot Works_In4 from to dname budget did Departments dname budget did name Departments ssn lot Employees Works_In4 Duration from to Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 2 Entity vs. Relationship (1/2)  Above ER diagram OK if a manager gets a separate discretionary budget for each dept.  What if a manager gets a (one) discretionary budget that covers all managed depts?  Redundancy: dbudget stored for each dept managed by manager.  Misleading: Suggests dbudget associated with department-mgr combination. Manages2 name dname budget did Employees Departments ssn lot dbudget since Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 2 Entity vs. Relationship (2/2)  Fixing the problem:  Create a new entity and assign “dbudget” to the entity rather than the realtionship.  Eliminates the redundancy  Clear association of “dbudget” to all dname budget did Departments Manages2 Employees name ssn lot since Managers dbudget ISA How would we change the ER model to represent one starting date for managing all departments? Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 2 Binary vs. Ternary Relationships  If each policy is owned by just 1 employee, and each dependent is tied to the covering policy, first diagram is inaccurate.  What are the additional constraints in the 2nd diagram? age pname Dependents Covers name Employees ssn lot Policies policyid cost Bad design Beneficiary age pname Dependents policyid cost Policies Purchaser name Employees ssn lot Better design Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 2 Binary vs. Ternary Relationships (Contd.)  Previous example illustrated a case when two binary relationships were better than one ternary relationship.  An example in the other direction: a ternary relation Contracts relates entity sets Parts, Departments and Suppliers, and has descriptive attribute qty. No combination of binary relationships is an adequate substitute:  S “can-supply” P, D “needs” P, and D “deals-with” S does not imply that D has agreed to buy P from S.  How do we record qty? Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 3 Aggregation vs Ternary budget did pid started_on pbudget dname until Departments Projects Sponsors Employees Monitors lot name ssn since  Without “until” attribute, we could have a Ternary relationship.  But can we say that each sponsorship is monitored by at most one employee? Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 3 Outline  Database Design and ER Diagram  Entities, Attributes, & Entity set  Relationships and relationship Set  Additional Features of the ER Model  Conceptual design with the ER Model  Conceptual Design for Large Enterprises  Case Study: Internet Shop Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 3 Case Study: B&N Hiring Dudes DB Co.  Requirements:  Books focusing on horse racing  Customers to browse catalog over the internet  Order books over the internet  Order contains: ISBN number, and quantity.  Shipment of whole order in one shot. If not all books in stock, then additional copies made.  For each book, catalog contains: ISBN, title, author, purchase price, sales price, and year book was published.  Records for customers are maintained: names and addresses.  New customers have to open an account first, and get CID.  Payment by credit card Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 3 Case Study (cont-d): Conceptual Schema address cid isbn price Yr_publish cname Customers Books Orders Order_date Ship_Date qty cardnum Qty_in_stock Author title How to handle?: 1. Same books in same day (qty), 2. diff books in same day (isbn) 3. Same books on different days  problem (order = cid/isbn). Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 3 Congratulations!!!! You have just designed your first Conceptual Database! Though not optimal yet  Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 3 Summary of Conceptual Design  Conceptual design follows requirements analysis,  Yields a high-level description of data to be stored  ER model popular for conceptual design  Constructs 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. Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 3 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  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.