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Entity Relationship Diagrams 1. Basic Elements and Rules.

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Presentation on theme: "Entity Relationship Diagrams 1. Basic Elements and Rules."— Presentation transcript:

1 Entity Relationship Diagrams 1. Basic Elements and Rules

2 Software Analysis & Design ER Model contains detailed descriptions of: What are the entities and relationships in the enterprise? What information about these entities and relationships should we store in the database? What are the integrity constraints or business rules that hold?

3 Data modelling(ERD) vs Process modelling(DFD) Process modelling (i.e. DFD) shows data stores, how, where, when data are used or changed in an IS Data modelling (i.e ER) shows definition, structure, and relationship within the data

4 Why data model is most important part of the statement of SW requirement? Characteristics of data captured during data modelling are crucial in design of DB, program, computer screen, and reports Data rather than processes are the most complex aspects of many modern IS so require a central role in structuring system requirement The characteristics of data (length, format, relationship) are reasonably permanen. The paths of data flow are quite dynamic. Structural information about data is essensial for automatic generation of programs

5 Conceptual Data Modeling and the E-R Diagram Goal Capture as much of the meaning of the data as possible A better design that is scalable and easier to maintain

6 Introduction to Entity- Relationship (E-R) Modeling Notation uses three main constructs Data entities Attributes Relationships Entity-Relationship (E-R) Diagram A detailed, logical representation of the entities, associations and data elements for an organization or business 10.6

7 Entity-Relationship (E-R) Modeling Key Terms Entity A person, place, object, event or concept in the user environment about which the organization wishes to maintain data Represented by a rectangle in E-R diagrams Attribute A named property or characteristic of an entity that is of interest to an organization 10.7

8 Entity-Relationship (E-R) Modeling Key Terms Candidate keys and identifiers Attribute (or combination of attributes) that uniquely identifies each instance of an entity type



11 E1 R E2 1 N … Notation Guide 1:N FOR E1:E2 IN R

12 ER Diagram Basics Relationship Attributes Entity Product Keeps Store descrip qty price pname manager Locations sname

13 Entity Real-world object distinguishable from other objects (e.g a student, car, job, subject, building...) An entity is described using a set of attributes The same entity may have different prominence in different scenario – In the Company database, an employees car is of lesser importance – In the Department of Transportations registration database, cars may be the most important concept – In both cases, cars will be represented as entities; but with different levels of detail

14 Attributes Key Attributes Value Sets of Attributes Null Valued Attributes Attribute Types – Composite Vs. Simple Attributes – Single-valued Vs. Multi-valued Attributes – Derived Vs. Stored AttributesNotation

15 Key Attributes Key (or uniqueness) constraints are applied to entity types Key attributes values are distinct for each individual entity. A key attribute has its name underlined inside the oval Multiple keys are possible EMPLOYEE SSN

16 Null Valued Attributes A particular entity may not have an applicable value for an attribute Type of Null Values – Not Applicable – Unknown – Missing

17 Composite Vs. Simple Attributes Composite attributes can be divided into smaller parts which represent simple attributes with independent meaning Simple Attribute: Aircraft-Type Complex Attribute: Aircraft-Location which is comprised of : Aircraft-Latitude Aircraft-Longitude Aircraft-Altitude Notation

18 Simple attributes can either be single-valued or multi-valued Single-valued: Gender = F Notation Multi-valued: Degree = {BSc, MTech} Notation Single Vs. Multivalued Attributes

19 Derived Vs. Stored Attributes Some attribute values can be derived from related attribute values: Age ® Date - B-day Y-Sal ® 12 * M-Sal EMPLOYEE M-sal B-days Y-sal Age Notation

20 Derived Vs. Stored Attributes Some attribute values can be derived from attributed values of related entities total-value = sum (qty * price) Order Item price qty Total-Value

21 Representing Attributes Parenthesis ( ) for composite attributes Brackets { } for multi-valued attributes Assume a person can have more than one residence and each residence can have multiple telephones {AddressPhone ({ Phone ( AreaCode,PhoneNum ) }, Address (StreetAddresss (Number, Street, AptNo), City,State,PostalCode) ) }

22 Entity-Relationship (E-R) Modeling Key Terms Relationship An association between the instances of one or more entities that is of interest to the organization Relationships are always labeled with verb phrases

23 Naming and Defining Relationships Relationship name is a verb phrase Avoid vague names Guidelines for defining relationships Definition explains what action is being taken and why it is important Give examples to clarify the action Optional participation should be explained Explain reasons for any explicit maximum cardinality

24 Relationship Degree The degree of a relationship type is the number of participating entity types – 2 entities: Binary Relationship 3 entities: Ternary Relationship – Same entity type could participate in multiple relationship types Part Supplier Supply Project Employees Departments Works_In Assigned_to Ternary Multiple Binary

25 Entity Roles Each entity type that participates in a relationship type plays a particular role in the relationship type The role name signifies the role that a participating entity from the entity type plays in each relationship instance, i.e. it explains what the relationship means Employees Works_In Departments employer worker RoleNames

26 Relationship Constraints What are Relationship Constraints ? Constraints on the relationship type limit the possible combination of entities that may participate in the corresponding relationship set

27 Possible Relations 1–to-1 (1 : 1) – Both entities can participate in only one relationship instance 1-to-Many, Many-to-1 (1 : N, N : 1) – One entity can participate in many relationship instances Many-to-Many (N: M) – Both entities can participate in many relationship instance to - Many Many - to to Many-to-Many

28 Example Cardinality Constraints How many Employees can work in a Department? One employee can work in only one department How many Employees can be employed by a Department? One department can employ many employees How many managers can a department have? One department can have only one manager How many departments can an employee manage? One employee can have manage only one department

29 Representing Relations One employee can work in only one department One department can employ many employees One department can have only one manager One employee can manage only one department N Employees Works_In Departments Manages

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