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Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke1 The Entity-Relationship Model Chapter 2
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke2 Steps of Database Design Requirements analysis Conceptual design: our focus for chapter 2 Logical database design: covered in chapter 3 Then: Refine schema, consider physical representation, consider security issues.
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke3 Overview of 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? A database `schema’ in the ER Model can be represented pictorially ( ER diagrams ). Can map an ER diagram into a relational schema (more later).
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke4 ER Model Basics Entity: 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. (Until we consider ISA hierarchies, anyway!) Each entity set has a key. Each attribute has a domain. Employees ssn name lot
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke5 ER Model Basics (Contd.) Relationship : Association among two or more entities. E.g., Attishoo works in Pharmacy department. 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 from E1, e2 from E2,..., en from En Same entity set can participate in different relationship sets, or in different “roles” in same set. lot dname budget did since name Works_In DepartmentsEmployees ssn Reports_To lot name Employees subor- dinate super- visor ssn
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke6 Key Constraints Consider Works_In: An employee can work in many departments; a dept can have many employees. Consider Manages: each dept has at most one manager, due to the key constraint. Note the same manager could manage any number of departments Many-to-Many 1-to-11-to ManyMany-to-1 dname budgetdid since lot name ssn Manages Employees Departments
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke7 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 Departments entity must appear in an instance of the Manages relationship. lot name dname budgetdid since name dname budgetdid since Manages since Departments Employees ssn Works_In
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke8 Weak Entities A weak entity can be identified uniquely only by considering the primary key of another ( owner ) entity (i.e. its entity set has no key of its own). 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. lot name age pname Dependents Employees ssn Policy cost
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke9 ISA (`is a’) Hierarchies Contract_Emps name ssn Employees lot hourly_wages ISA Hourly_Emps contractid hours_worked As in object-oriented languages, attributes are inherited. If we declare A ISA B, every A entity is also considered to be a B entity. Overlap 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.
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke10 Aggregation Used 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. * Aggregation vs. ternary relationship : Monitors is a distinct relationship with a descriptive attribute. Also, aggregation allows us to specify key constraints (e.g. each sponsorship monitored by at most 1 employee). budget did pid started_on pbudget dname until Departments Projects Sponsors Employees Monitors lot name ssn since
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke11 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? Should a relationship be 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. Gehrke12 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 allow a variable number of 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). Otherwise, address should be an attribute.
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke13 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. Gehrke14 Entity vs. Relationship First ER diagram OK if a manager gets a separate discretionary budget for each dept. What if a manager gets a 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 dname budget did Departments Manages2 Employees name ssn lot since Managersdbudget ISA This fixes the problem!
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke15 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 Beneficiary age pname Dependents policyid cost Policies Purchaser name Employees ssn lot Bad design Better design
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke16 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. Gehrke17 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. Note: There are many variations on ER model.
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke18 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.
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke19 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. Functional dependency (FD) information and normalization techniques are especially useful.