乔伊·F·乔治-现代系统分析与设计（第6版）hoffer_msad6e_ch08.pptVIP

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * ? 2011 Pearson Education, Inc. Publishing as Prentice Hall Relationships Relationship: an association between the instances of one or more entity types that is of interest to the organization Degree: the number of entity types that participate in a relationship * Chapter 8 ? 2011 Pearson Education, Inc. Publishing as Prentice Hall Conceptual Data Modeling and the E-R Model Unary relationship: a relationship between the instances of one entity type Also called a recursive relationship Binary relationship: a relationship between instances of two entity types Most common type of relationship encountered in data modeling Ternary relationship: a simultaneous relationship among instances of three entity types * Chapter 8 ? 2011 Pearson Education, Inc. Publishing as Prentice Hall * Chapter 8 Cardinalities in Relationships Cardinality: the number of instances of entity B that can (or must) be associated with each instance of entity A Minimum Cardinality The minimum number of instances of entity B that may be associated with each instance of entity A Maximum Cardinality The maximum number of instances of entity B that may be associated with each instance of entity A ? 2011 Pearson Education, Inc. Publishing as Prentice Hall * Chapter 8 Cardinalities in Relationships (Cont.) Mandatory vs. Optional Cardinalities Specifies whether an instance must exist or can be absent in the relationship. ? 2011 Pearson Education, Inc. Publishing as Prentice Hall * Chapter 8 FIGURE 8-14 Examples of cardinality constraints (a) Mandatory cardinalities (b) One optional, one mandatory cardinality (c) Optional cardinalities ? 2011 Pearson Education, Inc. Publishing as Prentice Hall Naming and Defining Relationships A relationship name is a verb phrase; avoid vague names. A relationship definition: Explains what action is to be taken and possibly why it is important. Gives examples to clarify the action. * Ch