tailieunhanh - Lecture Database Systems - Lecture 24

After completing this chapter, students will be able to: Classification of entities, attributes on relationships, structural constraints, multiplicity, binary/complex relationships, structural constraints, multiplicity, connection traps. | CSC271 Database Systems Lecture # 24 Summary: Previous Lecture Classification of entities Attributes on relationships Structural constraints Multiplicity Binary/Complex relationships Summary of Multiplicity Constraints Structural Constraints Multiplicity actually consists of two separate constraints known as Cardinality Participation Cardinality Describes maximum number of possible relationship occurrences for an entity participating in a given relationship type The cardinality of a binary relationship is what we previously referred to as a one-to-one (1:1), one-to-many (1:*), and many-to-many (*:*) The cardinality of a relationship appears as the maximum values for the multiplicity ranges on either side of the relationship Participation Determines whether all or only some entity occurrences participate in a relationship The participation constraint represents whether all entity occurrences are involved in a particular relationship (referred to as mandatory participation) or only some (referred to as optional participation) The participation of entities in a relationship appears as the minimum values for the multiplicity ranges on either side of the relationship Optional participation is represented as a minimum value of 0 while mandatory participation is shown as a minimum value of 1 Cardinality and Participation Problems with ER Models Problems may arise when designing a conceptual data (creating an ER) model called connection traps Normally occur due to a misinterpretation of the meaning of certain relationships Two main types of connection traps are: Fan traps Chasm traps Connection Traps In general, to identify connection traps we must ensure that the meaning of a relationship is fully understood and clearly defined If we do not understand the relationships we may create a model that is not a true representation of the ‘real world’ Fan Traps Where a model represents a relationship between entity types, but pathway between certain entity occurrences is ambiguous | CSC271 Database Systems Lecture # 24 Summary: Previous Lecture Classification of entities Attributes on relationships Structural constraints Multiplicity Binary/Complex relationships Summary of Multiplicity Constraints Structural Constraints Multiplicity actually consists of two separate constraints known as Cardinality Participation Cardinality Describes maximum number of possible relationship occurrences for an entity participating in a given relationship type The cardinality of a binary relationship is what we previously referred to as a one-to-one (1:1), one-to-many (1:*), and many-to-many (*:*) The cardinality of a relationship appears as the maximum values for the multiplicity ranges on either side of the relationship Participation Determines whether all or only some entity occurrences participate in a relationship The participation constraint represents whether all entity occurrences are involved in a particular relationship (referred to as mandatory participation) or only some