Chapter 7 Data Modeling Using the EntityRelationship ER Model Copyright 2011 Pearson Education Inc Publishing as Pearson Addison Wesley Chapter 7 Outline Using High Level Conceptual Data Models for Database Design A Sample Database Application Entity Types Entity Sets Attributes and Keys Relationship Types Relationship Sets Roles and Structural Constraints Weak Entity Types Copyright 2011 Ramez Elmasri and Shamkant Navathe Chapter 7 Outline cont d Refining the ER Design for the COMPANY Database ER Diagrams Naming Conventions and Design Issues Example of Other Notation UML Class Diagrams Relationship Types of Degree Higher than Two Copyright 2011 Ramez Elmasri and Shamkant Navathe Data Modeling Using the Entity Relationship ER Model Entity Relationship ER model Popular high level conceptual data model ER diagrams Diagrammatic notation associated with the ER model Unified Modeling Language UML Copyright 2011 Ramez Elmasri and Shamkant Navathe Using High Level Conceptual Data Models for Database Design Requirements collection and analysis Database designers interview prospective database users to understand and document data requirements Result data requirements Functional requirements of the application Copyright 2011 Ramez Elmasri and Shamkant Navathe Using High Level Conceptual Data Models cont d Conceptual schema Conceptual design Description of data requirements Includes detailed descriptions of the entity types relationships and constraints Transformed from high level data model into implementation data model Copyright 2011 Ramez Elmasri and Shamkant Navathe Using High Level Conceptual Data Models cont d Logical design or data model mapping Result is a database schema in implementation data model of DBMS Physical design phase Internal storage structures file organizations indexes access paths and physical design parameters for the database files specified Copyright 2011 Ramez Elmasri and Shamkant Navathe A Sample Database Application COMPANY Employees departments and projects Company is organized into departments Department controls a number of projects Employee store each employee s name Social Security number address salary gender and birth date Keep track of the dependents of each employee Copyright 2011 Ramez Elmasri and Shamkant Navathe Copyright 2011 Ramez Elmasri and Shamkant Navathe Entity Types Entity Sets Attributes and Keys ER model describes data as Entities Relationships Attributes Copyright 2011 Ramez Elmasri and Shamkant Navathe Entities and Attributes Entity Thing in real world with independent existence Attributes Particular properties that describe entity Types of attributes Composite versus simple atomic attributes Single valued versus multivalued attributes Stored versus derived attributes NULL values Complex attributes Copyright 2011 Ramez Elmasri and Shamkant Navathe Entities and Attributes cont d Copyright 2011 Ramez Elmasri and Shamkant Navathe Entity Types Entity Sets Keys and Value Sets Entity type Collection or set of entities that have the same attributes Copyright 2011 Ramez Elmasri and Shamkant Navathe Entity Types Entity Sets Keys and Value Sets cont d Key or uniqueness constraint Attributes whose values are distinct for each individual entity in entity set Key attribute Uniqueness property must hold for every entity set of the entity type Value sets or domain of values Specifies set of values that may be assigned to that attribute for each individual entity Copyright 2011 Ramez Elmasri and Shamkant Navathe Initial Conceptual Design of the COMPANY Database Copyright 2011 Ramez Elmasri and Shamkant Navathe Relationship Types Relationship Sets Roles and Structural Constraints Relationship When an attribute of one entity type refers to another entity type Represent references as relationships not attributes Copyright 2011 Ramez Elmasri and Shamkant Navathe Relationship Degree Degree of a relationship type Number of participating entity types Binary ternary Relationships as attributes Think of a binary relationship type in terms of attributes Copyright 2011 Ramez Elmasri and Shamkant Navathe Copyright 2011 Ramez Elmasri and Shamkant Navathe Role Names and Recursive Relationships Role names and recursive relationships Role name signifies role that a participating entity plays in each relationship instance Recursive relationships Same entity type participates more than once in a relationship type in different roles Must specify role name Copyright 2011 Ramez Elmasri and Shamkant Navathe Copyright 2011 Ramez Elmasri and Shamkant Navathe Constraints on Binary Relationship Types Cardinality ratio for a binary relationship Specifies maximum number of relationship instances that entity can participate in Participation constraint Specifies whether existence of entity depends on its being related to another entity Types total and partial Copyright 2011 Ramez Elmasri and Shamkant Navathe Attributes of Relationship Types Attributes of 1 1 or 1 N relationship types can be migrated to one entity type For a 1 N relationship type Relationship attribute can be migrated only to entity type on N side of relationship For M N relationship types Some attributes may be determined by combination of participating entities Must be specified as relationship attributes Copyright 2011 Ramez Elmasri and Shamkant Navathe Weak Entity Types Do not have key attributes of their own Identified by being related to specific entities from another entity type Identifying relationship Relates a weak entity type to its owner Always has a total participation constraint Copyright 2011 Ramez Elmasri and Shamkant Navathe Refining the ER Design for the COMPANY Database Change attributes that represent relationships into relationship types Determine cardinality ratio and participation constraint of each relationship type Copyright 2011 Ramez Elmasri and Shamkant Navathe ER Diagrams Naming Conventions and Design Issues Copyright 2011 Ramez Elmasri and Shamkant Navathe Proper Naming of Schema Constructs Choose names that convey meanings attached to different constructs in schema Nouns give rise to entity type names Verbs indicate names of relationship types Choose binary relationship names to make ER diagram readable from left to right and from top to bottom Copyright 2011 Ramez Elmasri and Shamkant Navathe Design Choices for ER Conceptual Design Model concept first as an attribute Refined into a relationship if attribute is a reference to another entity type Attribute that exists in several