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Comp3300/fall021 The Entity-Relationship Model Chapter 2 What are the steps in designing a database ? Why is the ER model used to create an initial design?

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Presentation on theme: "Comp3300/fall021 The Entity-Relationship Model Chapter 2 What are the steps in designing a database ? Why is the ER model used to create an initial design?"— Presentation transcript:

1 Comp3300/fall021 The Entity-Relationship Model Chapter 2 What are the steps in designing a database ? Why is the ER model used to create an initial design? What are the main concepts of ER model ? What are the guidelines for using ER model effectively? How does database design fit within the overall design framework for complex software within large enterprises?

2 Comp3300/fall022 Introduction to Database Design The entity-relationship( ER) data model allows us to describe the data in terms of objects and their relationships and is widely used To develop an initial database design. 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? The database design process can be divided into six steps. The ER model is most relevant to the first three steps.

3 Comp3300/fall023 Design process 1. Requirements Analysys What data is to be stored, what applications must be built on top of it; what are the most frequent operations to be performed, find out what the user wants from the database 2. Conceptual design: A high level description of data to be stored in the database. (ER Model is used at this stage.) This phase will be discussed in the rest of this chapter. 3. Logical Database Design: We must choose a DBMS to implement our DB design and convert the conceptual db design into a database schema in the data model of the chosen DBMS. ( This we will cover in chapter 3.)

4 Comp3300/fall024 Beyond ER Design We wil consider only Relational model, therefore A database `schema’ in the ER Model can be represented pictorially (ER diagrams). Can map an ER diagram into a relational schema ( conceptual schema sometime also called logical schema). 4. Schema refinement: analyze the collection of relations to identify potential problems and refine it. 5. Physical Database Design: Building indexes, clustering tables or redesigning some parts of db schemas. (out of our scope) 6. Application and Security Design:

5 Comp3300/fall025 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. - a minimal set of attributes whose values uniquely identify an entity in the entity set. There could be more than one candidate key. We select one of them to be primary key Each attribute has a domain. Employees ssn name lot

6 Comp3300/fall026 ER Model Basics (Contd.) v Relationship: Association among two or more entities. E.g., Attishoo works in Pharmacy department. v 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 E1,..., en En A relationship can also have descriptive attributes, which records information about the relationship. lot dname budget did since name Works_In DepartmentsEmployees ssn

7 Comp3300/fall027 ER Model Basics (Contd.) Reports_To lot name Employees super- visor ssn Same entity set could participate in different relationship sets, or in different “roles” in same set. Supervisor and subordinate are role indicators An instance of relationship set is a set of relationships Subor- dibnate

8 Comp3300/fall028 Key Constraints Consider Works_In: An employee can work in Many departments; a dept can have many employees. In contrast, each dept has At most one manager, according to the key constraint on Manages. Many-to-Many 1-to-11-to ManyMany-to-1 dname budgetdid since lot name ssn Manages Employees Departments

9 Comp3300/fall029 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 - ( thick line) vs. partial. Every did value in Departments table must appear in a row of the Manages table (with a non-null ssn value!) lot name dname budgetdid since name dname budgetdid since Manages since Departments Employees ssn Works_In

10 Comp3300/fall0210 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. pname: partial key lot name age pname Dependents Employees ssn Policy cost

11 Comp3300/fall0211 Class Hierarchies 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. 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.

12 Comp3300/fall0212 ISA (`is a’) Hierarchies Why we need to identify subclasses? - adding descriptive attributes that make sense only for a subclass - identify subset of entities that participate in some relations Contract_Emps name ssn Employees lot hourly_wages ISA Hourly_Emps contractid hours_worked

13 Comp3300/fall0213 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, can say that each sponsorship is monitored by at most one employee.

14 Comp3300/fall0214 budget did pid started_on pbudget dname until Departments Projects Sponsors Employees Monitors lot name ssn since Aggregation

15 Comp3300/fall0215 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.

16 Comp3300/fall0216 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).

17 Comp3300/fall0217 Entity vs. Attribute (Contd.) Works_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. name Employees ssn lot Works_In2 from to dname budget did Departments dname budget did name Departments ssn lot Employees Works_In3 Duration from to

18 Comp3300/fall0218 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 of dbudget, which is stored for each dept managed by the manager. Manages2 name dname budget did Employees Departments ssn lot dbudget since Employees since name dname budget did Departments ssn lot Mgr_Appts Manages3 dbudget apptnum Misleading: suggests dbudget tied to managed dept.

19 Comp3300/fall0219 Binary vs. Ternary Relationships If each policy is owned by just 1 employee: Key constraint on policies would mean policy can only cover 1 dependent! 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

20 Comp3300/fall0220 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?

21 Comp3300/fall0221 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.

22 Comp3300/fall0222 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.

23 Comp3300/fall0223 Summary of ER (Contd.) v 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. v Ensuring good database design: resulting relational schema should be analyzed and refined further. FD information and normalization techniques are especially useful.

24 Comp3300/fall0224 HomeWork #2 READING: Chapter II of your textbook (DMS),pp 25- 51 HOMEWORK Answer the following questions from your textbook, ( 53 for third edition ) Ex 2.1, 2.2, 2.4 SUBMITT: hard copy by the beginning of class Assigned 01/20/10 Due 01/27/10

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