CSC 742 Database Management Systems

Transcription

1 CSC 742 Database Management Systems Topic #4: Data Modeling Spring 2002 CSC 742: DBMS by Dr. Peng Ning 1 Phases of Database Design Requirement Collection/Analysis Functional Requirements Functional Analysis High-Level Transaction Spec. Application Program Design Transaction Implementation Database Requirements Conceptual Design Conceptual Schema Logical Design Logical Schema Physical Design Application Program Internal Schema Spring 2002 CSC 742: DBMS by Dr. Peng Ning 2 1

2 Part A: Entity-Relationship Model Spring 2002 CSC 742: DBMS by Dr. Peng Ning 3 What is ER Model About? Structure of the data Entities and relationships between (among) entities Constraints Conditions that the entities and relationships must satisfy. Key constraint Domain constraint Structural constraint Spring 2002 CSC 742: DBMS by Dr. Peng Ning 4 2

3 ER Concepts Entities Relationships Attributes Spring 2002 CSC 742: DBMS by Dr. Peng Ning 5 Attributes Atomic vs. composite Single- vs. multivalued Stored vs. derived Complex Attributes Spring 2002 CSC 742: DBMS by Dr. Peng Ning 6 3

4 Null Values Need Meanings not applicable unknown: missing or questionable existence Spring 2002 CSC 742: DBMS by Dr. Peng Ning 7 Entities Entity type (intension): e.g., Employee or Dept A collection of entities that have the same attributes Entity instance: e.g., Fred or Payroll Entity set (extension): e.g., {Fred, Bob,...} Spring 2002 CSC 742: DBMS by Dr. Peng Ning 8 4

5 Keys An intension corresponds to all possible extensions Superkey: a set of attributes that are unique for an entity type (i.e., for all possible extensions) Key: a minimal superkey fewer attributes won't be unique An entity type may have multiple keys Spring 2002 CSC 742: DBMS by Dr. Peng Ning 9 Relationships Relationship types: e.g., works-in Relationship instances: e.g., Fred works-in Payroll Spring 2002 CSC 742: DBMS by Dr. Peng Ning 10 5

6 Relationship Properties A relationship type associates entity types typically binary or ternary recursive may have attributes Spring 2002 CSC 742: DBMS by Dr. Peng Ning 11 Entity Types Participate in relationship types Have roles in those relationship types Spring 2002 CSC 742: DBMS by Dr. Peng Ning 12 6

7 Cardinality Cardinality constraints: number of relationship instances in which an entity instance may feature 1:1 1:N M:N Spring 2002 CSC 742: DBMS by Dr. Peng Ning 13 Achtung! Don't confuse 1:N with N:1 Some notations, especially for O-O modeling, write the cardinalities differently Spring 2002 CSC 742: DBMS by Dr. Peng Ning 14 7

8 Inferring Cardinalities We can construct paths between entity types These paths represent relationships composed from series of the existing relationships Their cardinalities can be inferred Spring 2002 CSC 742: DBMS by Dr. Peng Ning 15 Fan Traps Situations where the inferred, i.e., implied, cardinality is weaker than the actual cardinality Spring 2002 CSC 742: DBMS by Dr. Peng Ning 16 8

9 Participation Participation constraints: whether each entity instance must feature in some relationship instance total: yes partial: no Spring 2002 CSC 742: DBMS by Dr. Peng Ning 17 Chasm Traps When the composed relationship, i.e., path, has a weaker participation constraint than is actual Spring 2002 CSC 742: DBMS by Dr. Peng Ning 18 9

10 Weak Entity Types No key of its own attributes Must participate in a total relationship Another participant of the relationship becomes the owner Key = owner's key + partial key Spring 2002 CSC 742: DBMS by Dr. Peng Ning 19 Summary of ER Diagram Notations Entity Weak Entity Relationship Identifying Relationship Attribute Key Attribute Multi-valued Attribute Spring 2002 CSC 742: DBMS by Dr. Peng Ning 20 10

11 Summary of ER Diagram Notations Composite Attribute Derived Attribute E1 R E2 1 N E1 R E2 Total Participation of E2 in R Cardinality Ratio 1:N for E1:E2 In R R (min,max) E Structural Constraint (min,max) on Participation of E in R Spring 2002 CSC 742: DBMS by Dr. Peng Ning 21 Part B: Enhanced ER Model Spring 2002 CSC 742: DBMS by Dr. Peng Ning 22 11

12 Why Do We Need EER ER modeling is sufficient for representing many database schemas for traditional database applications. Recent applications require additional semantic data modeling concepts Class/subclass relationship Type inheritance Specialization and generalization. Spring 2002 CSC 742: DBMS by Dr. Peng Ning 23 Subclass-Superclass Subclasses: Further refinement (grouping) of a (super)class Attributes are inherited Class/subclass relationship is different from the relationship in ER modeling. Spring 2002 CSC 742: DBMS by Dr. Peng Ning 24 12

13 Specialization Specialization: The process of defining a set of subclasses of an entity type Top-down conceptual refinement Allows us to Define a set of subclasses of an entity type Establish additional specific attributes with each subclass Establish additional specific relationship types between each subclass and other entity types or other subclasses. Spring 2002 CSC 742: DBMS by Dr. Peng Ning 25 Generalization Generalization: creating a superclass by combining classes bottom-up conceptual synthesis Can be viewed as the inverse of the specialization process. Spring 2002 CSC 742: DBMS by Dr. Peng Ning 26 13

14 Classification Predicate-based: when a defining predicate determines the subclass of which a given instance is member Attributed-based: when the predicate applies only on an attribute User-defined: when the user decide the subclass membership Disjoint vs. overlapping Total vs. partial Spring 2002 CSC 742: DBMS by Dr. Peng Ning 27 Constraints on Specialization/Generalization Disjointness constraint The subclasses of the specialization must be disjoint. Specified by (d) Otherwise, the subclasses may overlap. Specified by (o) Spring 2002 CSC 742: DBMS by Dr. Peng Ning 28 14

15 Constraints on Specialization/Generalization Completeness constraint Total Specialization Partial Specialization Disjointness and completeness constraints are independent. Superclass identified from generalization is usually total. Spring 2002 CSC 742: DBMS by Dr. Peng Ning 29 Rules Delete from superclass subclasses delete from all Insert into predicate-based superclass insert where predicate holds Insert into total superclass insert into a subclass can't reasonably be done unless a predicate is specified Spring 2002 CSC 742: DBMS by Dr. Peng Ning 30 15

16 Structure Hierarchy: single inheritance Lattice: multiple inheritance Shared subclass Attribute inheritance Single inheritance: trivial Multiple inheritance Spring 2002 CSC 742: DBMS by Dr. Peng Ning 31 Union Types Category A subclass of the Union of some entity types A category has two or more super-classes Different from generalization. Spring 2002 CSC 742: DBMS by Dr. Peng Ning 32 16

17 Aggregation Combining objects to form a composite object. Three types of aggregations Aggregate attribute values of an object to form the object Represent an aggregation relationship as an ordinary relationship Combine objects that are related by a relationship into a higher-level aggregate object. Spring 2002 CSC 742: DBMS by Dr. Peng Ning 33 Association Associate objects from several independent classes. Not quite aggregation because deleting an entity instance doesn't destroy the instances it is composed of Spring 2002 CSC 742: DBMS by Dr. Peng Ning 34 17