Chapter 3: Modeling Data in the Organization PDF

Chapter 3: Modeling Data in the Organization Modern Database Management 8th Edition Jeffrey A. Hoffer, Mary B. Prescott, Fred R. McFadden © 2007 by Prentice Hall 1  Objectives Definition of terms Importance of data mode...

Chapter 3: Modeling Data in the Organization Modern Database Management 8th Edition Jeffrey A. Hoffer, Mary B. Prescott, Fred R. McFadden © 2007 by Prentice Hall 1  Objectives Definition of terms Importance of data modeling Write good names and definitions for entities, relationships, and attributes Distinguish unary, binary, and ternary relationships Model different types of attributes, entities, relationships, and cardinalities Draw E-R diagrams for common business situations Convert many-to-many relationships to associative entities Model time-dependent data using time stamps Chapter 3 © 2007 by Prentice Hall 2  Business Rules Statements that define or constrain some aspect of the business Assert business structure Control/influence business behavior Expressed in terms familiar to end users Automated through DBMS software Chapter 3 © 2007 by Prentice Hall 3  A Good Business Rule is: Declarative–what, not how Precise–clear, agreed-upon meaning Atomic–one statement Consistent–internally and externally Expressible–structured, natural language Distinct–non-redundant Business-oriented–understood by business people Chapter 3 © 2007 by Prentice Hall 4  A Good Data Name is: Related to business, not technical, characteristics Meaningful and self-documenting Unique Readable Composed of words from an approved list Repeatable Chapter 3 © 2007 by Prentice Hall 5  Data Definitions Explanation of a term or fact Term–word or phrase with specific meaning Fact–association between two or more terms Guidelines for good data definition Gathered in conjunction with systems requirements Accompanied by diagrams Iteratively created and refined Achieved by consensus Chapter 3 © 2007 by Prentice Hall 6  E-R Model Constructs Entities: Entity instance–person, place, object, event, concept (often corresponds to a row in a table) Entity Type–collection of entities (often corresponds to a table) Relationships: Relationship instance–link between entities (corresponds to primary key-foreign key equivalencies in related tables) Relationship type–category of relationship…link between entity types Attribute–property or characteristic of an entity or relationship type (often corresponds to a field in a table) Chapter 3 © 2007 by Prentice Hall 7 Sample E-R Diagram (Figure 3-1) Chapter 3 © 2007 by Prentice Hall 8 Basic E-R notation (Figure 3-2) Entity Attribute symbols symbols A special entity that is also a Relationship relationship symbols Relationship degrees specify number of entity types Relationship involved cardinalities specify how many of each entity type is allowed Chapter 3 © 2007 by Prentice Hall 9  What Should an Entity Be? SHOULD BE: An object that will have many instances in the database An object that will be composed of multiple attributes An object that we are trying to model SHOULD NOT BE: A user of the database system An output of the database system (e.g., a report) Chapter 3 © 2007 by Prentice Hall 10 Figure 3-4 Example of inappropriate entities System System user Inappropriate output entities Appropriate entities Chapter 3 © 2007 by Prentice Hall 11  Attributes Attribute–property or characteristic of an entity or relationahip type Classifications of attributes: Required versus Optional Attributes Simple versus Composite Attribute Single-Valued versus Multivalued Attribute Stored versus Derived Attributes Identifier Attributes Chapter 3 © 2007 by Prentice Hall 12  Identifiers (Keys) Identifier (Key)–An attribute (or combination of attributes) that uniquely identifies individual instances of an entity type Simple versus Composite Identifier Candidate Identifier–an attribute that could be a key…satisfies the requirements for being an identifier Chapter 3 © 2007 by Prentice Hall 13 Characteristics of Identifiers Will not change in value Will not be null No intelligent identifiers (e.g., containing locations or people that might change) Substitute new, simple keys for long, composite keys Chapter 3 © 2007 by Prentice Hall 14  Figure 3-7 A composite attribute An attribute broken into component parts Figure 3-8 Entity with multivalued attribute (Skill) and derived attribute (Years_Employed) Multivalued an employee can have Derived more than one skill from date employed and current Chapter 3 © 2007 by Prentice Hall date 15  Figure 3-9 Simple and composite identifier attributes The identifier is boldfaced and underlined Chapter 3 © 2007 by Prentice Hall 16  Figure 3-19 Simple example of time-stamping This attribute that is both multivalued and composite Chapter 3 © 2007 by Prentice Hall 17  More on Relationships Relationship Types vs. Relationship Instances The relationship type is modeled as lines between entity types…the instance is between specific entity instances Relationships can have attributes These describe features pertaining to the association between the entities in the relationship Two entities can have more than one type of relationship between them (multiple relationships) Associative Entity–combination of relationship and entity Chapter 3 © 2007 by Prentice Hall 18 Figure 3-10 Relationship types and instances a) Relationship type b) Relationship instances Chapter 3 © 2007 by Prentice Hall 19  Degree of Relationships Degree of a relationship is the number of entity types that participate in it Unary Relationship Binary Relationship Ternary Relationship Chapter 3 © 2007 by Prentice Hall 20  Degree of relationships – from Figure 3-2 Entities of One entity two different related to types related another of to each other Entities of three the same different types entity type related to each other Chapter 3 © 2007 by Prentice Hall 21  Cardinality of Relationships One-to-One Each entity in the relationship will have exactly one related entity One-to-Many An entity on one side of the relationship can have many related entities, but an entity on the other side will have a maximum of one related entity Many-to-Many Entities on both sides of the relationship can have many related entities on the other side Chapter 3 © 2007 by Prentice Hall 22  Cardinality Constraints Cardinality Constraints - the number of instances of one entity that can or must be associated with each instance of another entity Minimum Cardinality If zero, then optional If one or more, then mandatory Maximum Cardinality The maximum number Chapter 3 © 2007 by Prentice Hall 23 Figure 3-12 Examples of relationships of different degrees a) Unary relationships Chapter 3 © 2007 by Prentice Hall 24 Figure 3-12 Examples of relationships of different degrees (cont.) b) Binary relationships Chapter 3 © 2007 by Prentice Hall 25 Figure 3-12 Examples of relationships of different degrees (cont.) c) Ternary relationship Note: a relationship can have attributes of its own Chapter 3 © 2007 by Prentice Hall 26 Figure 3-17 Examples of cardinality constraints a) Mandatory cardinalities A patient history is A patient must have recorded for one and recorded at least one only one patient history, and can have many Chapter 3 © 2007 by Prentice Hall 27 Figure 3-17 Examples of cardinality constraints (cont.) b) One optional, one mandatory A project must be An employee can be assigned to at least assigned to any number one employee, and of projects, or may not be may be assigned to assigned to any at all many Chapter 3 © 2007 by Prentice Hall 28 Figure 3-17 Examples of cardinality constraints (cont.) a) Optional cardinalities A person is is married to at most one other person, or may not be married at all Chapter 3 © 2007 by Prentice Hall 29 Figure 3-21 Examples of multiple relationships a) Employees and departments Entities can be related to one another in more than one way Chapter 3 © 2007 by Prentice Hall 30 Figure 3-21 Examples of multiple relationships (cont.) b) Professors and courses (fixed lower limit constraint) Here, min cardinality constraint is 2 Chapter 3 © 2007 by Prentice Hall 31 gure 3-15a and 3-15b Multivalued attributes can be represented as relationships simple composite Chapter 3 © 2007 by Prentice Hall 32  Strong vs. Weak Entities, and Identifying Relationships Strong entities exist independently of other types of entities has its own unique identifier identifier underlined with single-line Weak entity dependent on a strong entity (identifying owner)…cannot exist on its own does not have a unique identifier (only a partial identifier) Partial identifier underlined with double-line Entity box has double line Identifying relationship links strong entities to weak entities Chapter 3 © 2007 by Prentice Hall 33  Identifying relationship Strong entity Weak entity Chapter 3 © 2007 by Prentice Hall 34  Associative Entities An entity–has attributes A relationship–links entities together When should a relationship with attributes instead be an associative entity? All relationships for the associative entity should be many The associative entity could have meaning independent of the other entities The associative entity preferably has a unique identifier, and should also have other attributes The associative entity may participate in other relationships other than the entities of the associated relationship Ternary relationships should be converted to associative entities Chapter 3 © 2007 by Prentice Hall 35 Figure 3-11a A binary relationship with an attribute Here, the date completed attribute pertains specifically to the employee’s completion of a course…it is an attribute of the relationship Chapter 3 © 2007 by Prentice Hall 36 Figure 3-11b An associative entity (CERTIFICATE) Associative entity is like a relationship with an attribute, but it is also considered to be an entity in its own right. Note that the many-to-many cardinality between entities in Figure 3-11a has been replaced by two one-to-many relationships with the associative entity. Chapter 3 © 2007 by Prentice Hall 37 Figure 3-13c An associative entity – bill of materials structur This could just be a relationship with attributes…it’s a judgment call Chapter 3 © 2007 by Prentice Hall 38 Figure 3-18 Ternary relationship as an associative entity Chapter 3 © 2007 by Prentice Hall 39  Microsoft Visio Notation for Pine Valley Furniture E-R diagram Different modeling software tools may have different notation for the same constructs Chapter 3 © 2007 by Prentice Hall 40