02010 ppt ch04
- 1. Database Systems: Design, Implementation, and Management Eighth Edition Chapter 4 Entity Relationship (ER) Modeling
- 2. Database Systems, 8th Edition 2 Objectives • In this chapter, you will learn: – The main characteristics of entity relationship components – How relationships between entities are defined, refined, and incorporated into the database design process – How ERD components affect database design and implementation – That real-world database design often requires the reconciliation of conflicting goals
- 3. Database Systems, 8th Edition 3 The Entity Relationship (ER) Model • ER model forms the basis of an ER diagram • ERD represents conceptual database as viewed by end user • ERDs depict database’s main components: – Entities – Attributes – Relationships
- 4. Database Systems, 8th Edition 4 Entities • Refers to entity set and not to single entity occurrence • Corresponds to table and not to row in relational environment • In Chen and Crow’s Foot models, entity represented by rectangle with entity’s name • Entity name, a noun, written in capital letters
- 5. Database Systems, 8th Edition 5 Attributes • Characteristics of entities • Chen notation: attributes represented by ovals connected to entity rectangle with a line – Each oval contains the name of attribute it represents • Crow’s Foot notation: attributes written in attribute box below entity rectangle
- 7. Database Systems, 8th Edition 7 Attributes (continued) • Required attribute:must have a value are bold • Optional attribute: may be left empty • Domain: set of possible values for an attribute – Attributes may share a domain • Identifiers: one or more attributes that uniquely identify each entity instance • Composite identifier: primary key composed of more than one attribute
- 9. Database Systems, 8th Edition 9 Attributes (continued) • Composite attribute can be subdivided • Simple attribute cannot be subdivided • Single-value attribute can have only a single value • Multivalued attributes can have many values
- 11. Implementing Multivalued Attributes Database Systems, 8th Edition 11
- 13. Database Systems, 8th Edition 13 Attributes (continued) • M:N relationships and multivalued attributes should not be implemented – Create several new attributes for each of the original multivalued attributes components – Create new entity composed of original multivalued attributes components • Derived attribute: value may be calculated from other attributes – Need not be physically stored within database
- 15. Database Systems, 8th Edition 15 Relationships • Association between entities • Participants are entities that participate in a relationship • Relationships between entities always operate in both directions • Relationship can be classified as 1:M • Relationship classification is difficult to establish if only one side of the relationship is known
- 16. Database Systems, 8th Edition 16 Connectivity and Cardinality • Connectivity – Describes the relationship classification • Cardinality – Expresses minimum and maximum number of entity occurrences associated with one occurrence of related entity • Established by very concise statements known as business rules
- 18. Database Systems, 8th Edition 18 Existence Dependence • Existence dependence – Entity exists in database only when it is associated with another related entity occurrence • Existence independence – Entity can exist apart from one or more related entities – Sometimes such an entity is referred to as a strong or regular entity
- 19. Database Systems, 8th Edition 19 Relationship Strength • Weak (non-identifying) relationships – Exists if PK of related entity does not contain PK component of parent entity • Strong (identifying) relationships – Exists when PK of related entity contains PK component of parent entity
- 22. Database Systems, 8th Edition 22 Weak Entities • Weak entity meets two conditions 1. Existence-dependent 2. Primary key partially or totally derived from parent entity in relationship • Database designer determines whether an entity is weak based on business rules
- 25. Database Systems, 8th Edition 25 Relationship Participation • Optional participation – One entity occurrence does not require corresponding entity occurrence in particular relationship • Mandatory participation – One entity occurrence requires corresponding entity occurrence in particular relationship
- 29. Database Systems, 8th Edition 29 Relationship Degree • Indicates number of entities or participants associated with a relationship • Unary relationship “recursive relationship” – Association is maintained within single entity • EX: an employee within the EMPLOYEE entity is the manager for one or more employees within that entity. • EMPLOYEE requires another EMPLOYEE to be the manager • that is, EMPLOYEE has a relationship with itself • Binary relationship – Two entities are associated • Ternary relationship – Three entities are associated
- 32. Database Systems, 8th Edition 32 Recursive Relationships • Relationship can exist between occurrences of the same entity set – Naturally found within unary relationship
- 35. Database Systems, 8th Edition 35 Associative (Composite) Entities • Also known as bridge entities • Used to implement M:N relationships • Composed of primary keys of each of the entities to be connected • May also contain additional attributes that play no role in connective process
- 38. Database Systems, 8th Edition 38 Developing an ER Diagram • Database design is an iterative process – Create detailed narrative of organization’s description of operations – Identify business rules based on description of operations – Identify main entities and relationships from business rules – Develop initial ERD – Identify attributes and primary keys that adequately describe entities – Revise and review ERD
- 50. Database Systems, 8th Edition 50 Database Design Challenges: Conflicting Goals • Database designers must make design compromises – Conflicting goals: design standards, processing speed, information requirements • Important to meet logical requirements and design conventions • Design of little value unless it delivers all specified query and reporting requirements • Some design and implementation problems do not yield “clean” solutions
- 52. Database Systems, 8th Edition 52 Summary • Entity relationship (ER) model – Uses ERD to represent conceptual database as viewed by end user – ERM’s main components: • Entities • Relationships • Attributes – Includes connectivity and cardinality notations
- 53. Database Systems, 8th Edition 53 Summary (continued) • Connectivities and cardinalities are based on business rules • M:N relationship is valid at conceptual level – Must be mapped to a set of 1:M relationships • ERDs may be based on many different ERMs • Database designers are often forced to make design compromises