11/3/2019 Er Diagram Tutorial Pdf
Of the ER model In addition, EER includes: Subclasses and superclasses Specialization and generalization Category or union type Attribute and relationship inheritance. Chapter 2: Entity-Relationship Model! Relationship Sets! Design Issues! Mapping Constraints! Extended E-R Features! Design of an E-R Database Schema! Reduction of an E-R Schema to Tables Database System Concepts 2.2 ©Silberschatz, Korth and Sudarshan Entity Sets! A database can be modeled as:' a collection of. An Entity Relationship (ER) Diagram is a type of flowchart that illustrates how “entities” such as people, objects or concepts relate to each other within a system. ER Diagrams are most often used to design or debug relational databases in the fields of software engineering, business information systems, education and research. Entity and Attribute Entity Entity class (entity set) is a structural description of things that share common attributes Entity instance is the occurrence of a particular entity Attribute Describes an entity class All entity instances of a given entity class have the same attributes, but. Let us now learn how the ER Model is represented by means of an ER diagram. Any object, for example, entities, attributes of an entity, relationship sets, and attributes of relationship sets, can be represented with the help of an ER diagram. Entities are represented by means of rectangles. Rectangles are named with the entity set they represent.
What is the ER Model?
The ER or (Entity Relational Model) is a high-level conceptual data model diagram. Entity-Relation model is based on the notion of real-world entities and the relationship between them.
ER modeling helps you to analyze data requirements systematically to produce a well-designed database. So, it is considered a best practice to complete ER modeling before implementing your database.
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In this tutorial, you will learn-
History of ER models
ER diagrams are a visual tool which is helpful to represent the ER model. It was proposed by Peter Chen in 1971 to create a uniform convention which can be used for relational database and network. He aimed to use an ER model as a conceptual modeling approach.
What is ER Diagrams?
Entity relationship diagram displays the relationships of entity set stored in a database. In other words, we can say that ER diagrams help you to explain the logical structure of databases. At first look, an ER diagram looks very similar to the flowchart. However, ER Diagram includes many specialized symbols, and its meanings make this model unique.
Facts about ER Diagram Model:
Why use ER Diagrams?
Here, are prime reasons for using the ER Diagram
Components of the ER Diagram
This model is based on three basic concepts:
Example
For example, in a University database, we might have entities for Students, Courses, and Lecturers. Students entity can have attributes like Rollno, Name, and DeptID. They might have relationships with Courses and Lecturers.
WHAT IS ENTITY?
A real-world thing either living or non-living that is easily recognizable and nonrecognizable. It is anything in the enterprise that is to be represented in our database. It may be a physical thing or simply a fact about the enterprise or an event that happens in the real world.
An entity can be place, person, object, event or a concept, which stores data in the database. The characteristics of entities are must have an attribute, and a unique key. Every entity is made up of some 'attributes' which represent that entity.
Examples of entities:
Notation of an Entity
Entity set:
Student
An entity set is a group of similar kind of entities. It may contain entities with attribute sharing similar values. Entities are represented by their properties, which also called attributes. All attributes have their separate values. For example, a student entity may have a name, age, class, as attributes.
Example of Entities:
A university may have some departments. All these departments employ various lecturers and offer several programs.
Some courses make up each program. Students register in a particular program and enroll in various courses. A lecturer from the specific department takes each course, and each lecturer teaches a various group of students.
Relationship
Relationship is nothing but an association among two or more entities. E.g., Tom works in the Chemistry department.
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Entities take part in relationships. We can often identify relationships with verbs or verb phrases.
For example:
Weak Entities
A weak entity is a type of entity which doesn't have its key attribute. It can be identified uniquely by considering the primary key of another entity. For that, weak entity sets need to have participation.
In aboe example, 'Trans No' is a discriminator within a group of transactions in an ATM.
Let's learn more about a weak entity by comparing it with a Strong Entity
Attributes
It is a single-valued property of either an entity-type or a relationship-type.
For example, a lecture might have attributes: time, date, duration, place, etc.
An attribute is represented by an Ellipse
Cardinality
Defines the numerical attributes of the relationship between two entities or entity sets.
Different types of cardinal relationships are:
1.One-to-one:
One entity from entity set X can be associated with at most one entity of entity set Y and vice versa.
Example: One student can register for numerous courses. However, all those courses have a single line back to that one student.
2.One-to-many:
One entity from entity set X can be associated with multiple entities of entity set Y, but an entity from entity set Y can be associated with at least one entity.
For example, one class is consisting of multiple students.
3. Many to One
More than one entity from entity set X can be associated with at most one entity of entity set Y. However, an entity from entity set Y may or may not be associated with more than one entity from entity set X.
For example, many students belong to the same class.
4. Many to Many:
One entity from X can be associated with more than one entity from Y and vice versa.
For example, Students as a group are associated with multiple faculty members, and faculty members can be associated with multiple students.
ER- Diagram Notations
ER- Diagram is a visual representation of data that describe how data is related to each other.
Steps to Create an ERD
Following are the steps to create an ERD.
Let's study them with an example:
Step 1) Entity Identification
We have three entities
Step 2) Relationship Identification
We have the following two relationships
Step 3) Cardinality Identification
For them problem statement we know that,
Step 4) Identify Attributes
You need to study the files, forms, reports, data currently maintained by the organization to identify attributes. You can also conduct interviews with various stakeholders to identify entities. Initially, it's important to identify the attributes without mapping them to a particular entity.
Once, you have a list of Attributes, you need to map them to the identified entities. Ensure an attribute is to be paired with exactly one entity. If you think an attribute should belong to more than one entity, use a modifier to make it unique.
Once the mapping is done, identify the primary Keys. If a unique key is not readily available, create one.
For Course Entity, attributes could be Duration, Credits, Assignments, etc. For the sake of ease we have considered just one attribute.
Step 5) Create the ERD
A more modern representation of ERD Diagram
Best Practices for Developing Effective ER Diagrams
SummaryEr Diagram Tutorial Pdf In Tamil
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