ETEC 2601 Database Systems

Transcription

1 ETEC 2601 Database Systems Chapter 5: Data Modeling With The Entity-Relationship Model Copyright J. B. Gallaher

2 The Data Model The design of a database Is the way the user conceives the information to be used in their system We do the data modeling first to get an understanding of how the database needs to be designed It is easier to change the data model than it is to change the database design Properly applied, the data model will help us work through problems in applying database concepts to the information

3 The Entity-Relationship Model The Entity-Relationship Model models the real world as a set of trackable things that have definable characteristics. Things in the real world are related to and sometimes dependant on other things. The E-R model defines a consistent way of defining how to categorize these things and how they interact. This is called the Schema Graphical symbols are used to define the model of an application so that it can be converted into a database. This is called an E-R Diagram or UML Diagram

4 The Entity-Relationship Model (continued) There are variations of the E-R model that attempt to continue to refine the way the data is modeled. The E-R model was first. It defined the way entities were defined and how the relationships were formed. An Extended E-R model evolved which now included the concept of Subtypes. (A programmer, accountant, secretary, engineer, etc. are subtypes of an employee.) IDEFX1 then evolved to include the concept of Domains (allowable values) and a more precise way to graphically depict a model Unified Modeling Language (UML) supports the object-oriented methodology

5 Entities Entities, Attributes, and Identifiers Entity = trackable thing. Sometimes referred to as an Entity Class. An Entity Instance is the occurrence of a particular entity. Student Student Number First Name Middle Initial Last Name Address City State Zip Major Student Number is an Identifier. A characteristic of the entity that is unique to each instance of that thing. (e.g. Student Number, Social Security Number, Invoice Number, etc.) Composite Identifiers consist of two or more attributes. Attributes = Characteristics of all instances of members of this entity class. This is the definition of the entity. (Name and attributes) Instances of a Student thing Irving V Schwartz 1234 Anywhere Rd. Portsmouth Oh Sherman D Tank 666 Mark O Devil Rd. Portsmouth Oh 45662

6 Relationships Associate one entity with another The degree of a relationship is determined by the number of tables involved two tables = binary, three tables = ternary Student Student Number First Name Middle Initial Last Name Address City State Zip Major Students take classes This is a has a relationship Class Class ID Name Description Lecture Hours Lab Hours Three types of relationships: 1:1 One-To-One Student -> Truck 1:Many One-To-Many Student -> Classes Many:Many Many-To-Many Student -> Major

7 Relationships (continued) Graphical representation of Relationships: Student 1:1 Truck One-To-One Relationship Class 1:N Students One-To-Many Relationship Student M:N Major Many-To-Many Relationship

8 Entities and Tables Entities are things Tables are constructed to hold information about these things We can express a relationship between entities without using foreign keys In the conceptual stage, entities are easier to deal with since the relationships are more ambiguous As the design is firmed-up the relationships can then be expressed in terms of keys and identifiers The entities can then be distilled into table form

9 Cardinality Cardinality means Count It is expressed as a number There are two types of cardinality: Maximum the maximum number of instances of an entity that can participate in a relationship Minimum the minimum number of instances that must participate in a relationship

10 Maximum Cardinality Maximum Cardinality is the maximum number of entity instances than can participate in a relationship Three types of maximum cardinality:

11 Parent and Child Entities In a one-to-many relationship The entity on the one side of the relationship is called the parent entity or just the parent The entity on the many side of the relationship is called the child entity of just the child These are known as Has-A-Relationships Each entity instance has a relationship with another entity instance Parent Entity Child Entity

12 Minimum Cardinality Minimum Cardinality is the minimum number of entity instances that mustparticipate in a relationship. Minimum cardinality is generally stated as either zero or one; If zero then participation in the relationship is optional If one then the participation in the relationship is mandatory An oval next to an entity means the entity is optional A vertical hash mark means the entity is mandatory

13 ID-Dependent Entities A child entity whose identifier includes the identifier of its parent entity It is a logical extension or sub-unit of the parent The minimum cardinality from the ID-dependent entity to the parent is always one.

14 Weak Entities An entity whose existence depends on another entity Weak Entity - Exists only if Employee Exists. Must Logically depend on the required entity Employee Identifier: Employee No. 1:N Non-ID Dependent Relationship Dependent (weak entity does not contain the ID of the strong entity it has its own identifier) Identifier: Social Security No. Rounded corners indicate weak entity relationship and which is the weak entity. Dorm 1:N Room Number Identifier: Building Name ID Dependent Relationship (weak entity contains the identifier of the strong entity) Identifier: Building Name & Apartment Number

15 ID-Dependent and Weak Entities An ID-Dependent entity is an entity whose identifier includes the identifier of another entity Identifying relationships are used to represent ID-Dependent entities A weak entity is an entity whose existence depends on antoher entity All ID-Dependent entities are weak Some entities are weak, but not ID-Dependent.

16 The Extended E-R Model (continued) Showing Attributes In E-R Diagrams Student Number Major ISBN Title First Name Student 1:N Books Middle Initial Author Last Name Address Edition Attributes are the characteristics that describe each entity. They are the columns in the table that that are qualities common to all of the instances of that entity.

17 Subtype Entities Subtype Entities Last Name Student First Name Student Number Major Є Є Є 1 Indicates that a student can be only one of the subtypes Undergraduate Graduate Doctoral Advisor Thesis Topic Concentration Area Dissertation Topic Indicates a subtype relationship Doctoral Advisor Subtype Entities form an IS-A relationship. (An undergraduate student is a student as is the Graduate Student and the Doctoral Student. Subtypes are used to avoid value-inappropriate nulls.

18 Subtypes: Exclusive or Inclusive If subtypes are exclusive, one supertype relates to at most one subtype If subtypes are inclusive, one supertype can relate to one or more subtypes

19 Patterns in Forms, Reports, and Entity- Relationship Models We can develop data models of existing systems by studying the documents available Forms and reports usually show entities and their relationships Forms and reports can also be used to validate the data model The structure of forms and reports usually falls into predictable patters

20 Strong Entity Patterns 1:1 Strong Entity Relationships

21 Strong Entity Patterns 1:N Strong Entity Relationships

22 Strong Entity Patterns N:M Strong Entity Relationships

23 ID-Dependent Relationships Association Pattern Different from N:M strong relationships in that they contain additional information Note that Price is not part of either the Part or Company definitions. It appears in the association table.

24 ID-Dependent Relationships Multivalued Attribute Pattern Some attributes can have multiple values (e.g. phone numbers)

25 The Archtype/Instance Pattern ID-dependent child entity is an instance of a logical parent e.g. painting -> print e.g. class -> section Note that the identifier of the parent is part of the composite identifier of the dependent child.

26 The Archtype/Instance Pattern Non-ID-dependent child entity is an instance of a logical parent Note that the identifier of the parent is not part of the identifier of the dependent child. A surrogate key is used to identify it.

27 Mixed Patterns The Line-Item Pattern

28 Mixed Patterns Other Mixed Patterns A strong entity has a multivalued composite group, and One of the elements of the composite group is an identifier of another strong entity.

29 Mixed Patterns The For-Use-By Pattern Forms that contain blocks grayed-out and marked For Use by someone/something Only

30 Recursive Relationships Occurs when an entity has a relationship to itself 1:1 Recursive Relationship

31 Recursive Patterns 1:N Recursive Relationship

32 Recursive Patterns N:M Recursive Relationship