What is ER Diagram?ER Diagram stands for Entity Relationship Diagram, also known as ERD is a diagram that displays the relationship of entity sets stored in a database. In other words, ER diagrams help to explain the logical structure of databases. ER diagrams are created based on three basic concepts: entities, attributes and relationships. Show
ER Diagrams contain different symbols that use rectangles to represent entities, ovals to define attributes and diamond shapes to represent relationships. 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. The purpose of ER Diagram is to represent the entity framework infrastructure. Entity Relationship Diagram ExampleWhat is ER Model?ER Model stands for Entity Relationship Model is a high-level conceptual data model diagram. ER model helps to systematically analyze data requirements to produce a well-designed database. The ER Model represents real-world entities and the relationships between them. Creating an ER Model in DBMS is considered as a best practice before implementing your database. 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. In this Entity Relationship Diagram tutorial, you will learn-
History of ER modelsER diagrams are visual tools that are helpful to represent the ER model. Peter Chen proposed ER Diagram in 1971 to create a uniform convention that can be used for relational databases and networks. He aimed to use an ER model as a conceptual modeling approach. Why use ER Diagrams?Here, are prime reasons for using the ER Diagram
Facts about ER Diagram ModelNow in this ERD Diagram Tutorial, let’s check out some interesting facts about ER Diagram Model:
ER Diagrams Symbols & NotationsEntity Relationship Diagram Symbols & Notations mainly contains three basic symbols which are rectangle, oval and diamond to represent relationships between elements, entities and attributes. There are some sub-elements which are based on main elements in ERD Diagram. ER Diagram is a visual representation of data that describes how data is related to each other using different ERD Symbols and Notations. Following are the main components and its symbols in ER Diagrams:
Components of the ER DiagramThis model is based on three basic concepts:
ER Diagram Examples 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. Components of the ER DiagramWHAT 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 is nothing but an association among two or more entities. E.g., Tom works in the Chemistry department. Entities take part in relationships. We can often identify relationships with verbs or verb phrases. For example:
Weak EntitiesA 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 above ER Diagram examples, “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
AttributesIt 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 in ER Diagram examples, is represented by an Ellipse
CardinalityDefines 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. How to Create an Entity Relationship Diagram (ERD)Now in this ERD Diagram Tutorial, we will learn how to create an ER Diagram. Following are the steps to create an ER Diagram: Steps to Create an ER DiagramLet’s study them with an Entity Relationship Diagram Example: Step 1) Entity IdentificationWe have three entities
Step 2) Relationship IdentificationWe have the following two relationships
Step 3) Cardinality IdentificationFor them problem statement we know that,
Step 4) Identify AttributesYou 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 DiagramA more modern representation of Entity Relationship Diagram Example Best Practices for Developing Effective ER DiagramsHere are some best practice or example for Developing Effective ER Diagrams.
Summary
What are the symbols used in entity relationship diagram?Entity Relationship Diagram Symbols & Notations mainly contains three basic symbols which are rectangle, oval and diamond to represent relationships between elements, entities and attributes.
What is the symbol for many in a oneMany-to-Many relation: A one-to-many relationship is represented by adding 'M' near the entity at left hand side of relation and 'N' is written near the entity at right side. Other type of notation will have three arrow kinds of lines at both sides of relation as shown below.
How do you show manyGraphically, the many to many relationship is usually represented in a logical diagram with crow's foot notation. In a relational database, this relationship is then usually implemented using a join table, otherwise known as a junction or associative table with two one-to-many relationships.
How do you represent relationships in ERD?In ER diagram, relationship type is represented by a diamond and connecting the entities with lines. A set of relationships of same type is known as relationship set.
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