An Entity Relationship Diagram (ERD) is a visual representation of different data using conventions that describe how these data are related to each other.
ER Diagrams Usage:-
While able to describe just about any system, ER diagrams are most often associated with complex databases that are used in software engineering and IT networks. In particular, ER diagrams are frequently used during the design stage of a development process in order to identify different system elements and their relationships with each other. For example, an inventory software used in a retail shop will have a database that monitors elements such as purchases, item, item type, item source and item price.
History of ER Diagrams:-
ER diagrams are visual tools that are used in the Entity-Relationship model initially proposed by Peter Chen in 1976 to create a uniform convention that considers both relational database and network views. Chen envisioned the ER model as a conceptual modeling approach that views real world data as systems of entities and relationships. Entities are data objects that maintain different relationships with each other. Additionally, entities are also described further using attributes.
Since 1976, the ER model has been expanded and is sometimes used in business management, product development, and strategy formulations. However, database design remains its primary application.
Elements in ER diagrams :-
There are three basic elements in an ER Diagram: entity, attribute, relationship. There are more elements which are based on the main elements. They are weak entity, multivalued attribute, derived attribute, weak relationship and recursive relationship. Cardinality and ordinality are two other notations used in ER diagrams to further define relationships.
Entity :-
An entity can be a person, place, event, or object that is relevant to a given system. For example, a school system may include students, teachers, major courses, subjects, fees, and other items. Entities are represented in ER diagrams by a rectangle and named using singular nouns.
Weak Entity :-
A weak entity is an entity that depends on the existence of another entity. In more technical terms it can defined as an entity that cannot be identified by its own attributes. It uses a foreign key combined with its attributed to form the primary key. An entity like order item is a good example for this. The order item will be meaningless without an order so it depends on the existence of order.
Attribute :-
An attribute is a property, trait, or characteristic of an entity, relationship, or another attribute. For example, the attribute Inventory Item Name is an attribute of the entity Inventory Item. An entity can have as many attributes as necessary. Meanwhile, attributes can also have their own specific attributes. For example, the attribute “customer address” can have the attributes number, street, city, and state. These are called composite attributes. Note that some top level ER diagrams do not show attributes for the sake of simplicity. In those that do, however, attributes are represented by oval shapes.
Attributes in ER diagrams, note that an attribute can have its own attributes ( composite attribute )
Multivalued Attribute :-
If an attribute can have more than one value it is called an multivalued attribute. It is important to note that this is different to an attribute having its own attributes. For example a teacher entity can have multiple subject values.
Derived Attribute :-
An attribute based on another attribute. This is found rarely in ER diagrams. For example for a circle the area can be derived from the radius.
Relationship :-
A relationship describes how entities interact. For example, the entity “carpenter” may be related to the entity “table” by the relationship “builds” or “makes”. Relationships are represented by diamond shapes and are labeled using verbs.
Recursive Relationship :-
If the same entity participates more than once in a relationship it is known as a recursive relationship. In the below example an employee can be a supervisor and be supervised, so there is a recursive relationship.
Cardinality and Ordinality :-
These two further defines relationships between entities by placing the relationship in the context of numbers. In an email system, for example, one account can have multiple contacts. The relationship in this case follows a “one to many” model. There are number of notations used to present cardinality in ER diagrams. Chen, UML, Crow’s foot, Bachman are some of the popular notations. Creately supports Chen, UML and Crow’s foot notations.
Benefits of ER diagrams : -ER diagrams constitute a very useful framework for creating and manipulating databases. First, ER diagrams are easy to understand and do not require a person to undergo extensive training to be able to work with it efficiently and accurately. This means that designers can use ER diagrams to easily communicate with developers, customers, and end users, regardless of their IT proficiency. Second, ER diagrams are readily translatable into relational tables which can be used to quickly build databases. In addition, ER diagrams can directly be used by database developers as the blueprint for implementing data in specific software applications. Lastly, ER diagrams may be applied in other contexts such as describing the different relationships and operations within an organization.
How to Draw ER Diagrams :-
Because ER diagrams are simple enough to understand, just about anyone can create them. However, two different ER diagrams describing the same system may still be radically different in terms of their simplicity, completeness, and efficiency at communicating the system. In other words, there are good ER diagrams and there are poor ones.
Because this ER tutorial focuses on beginners below are some tips that will help you build effective ER diagrams:
1. Identify all the relevant entities in a given system and determine the relationships among these entities.
2. An entity should appear only once in a particular diagram.
3. Provide a precise and appropriate name for each entity, attribute, and relationship in the diagram. Terms that are simple and familiar always beats vague, technical-sounding words. In naming entities, remember to use singular nouns. However, adjectives may be used to distinguish entities belonging to the same class (part-time employee and full time employee, for example). Meanwhile attribute names must be meaningful, unique, system-independent, and easily understandable.
4. Remove vague, redundant or unnecessary relationships between entities.
5. Never connect a relationship to another relationship.
6. Make effective use of colors. You can use colors to classify similar entities or to highlight key areas in your diagrams.
How to Create an Entity Relationship Diagram
Here are some best practice tips for constructing an ERD:
Identify the entities. The first step in making an ERD is to identify all of the entities you will use.
An entity is nothing more than a rectangle with a description of something that your system stores information about.
This could be a customer, a manager, an invoice, a schedule, etc. Draw a rectangle for each entity you can think of on your page.
Keep them spaced out a bit.
ERD entities
Identify relationships. Look at two entities, are they related? If so draw a solid line connecting the two entities.
Describe the relationship. How are the entities related? Draw an action diamond between the two entities on the line you just added.
In the diamond write a brief description of how they are related.
Add attributes. Any key attributes of entities should be added using oval-shaped symbols.
Complete the diagram. Continue to connect the entities with lines, and adding diamonds to describe each relationship
until all relationships have been described. Each of your entities may not have any relationships, some may have multiple relationships.
ER Diagrams Usage:-
While able to describe just about any system, ER diagrams are most often associated with complex databases that are used in software engineering and IT networks. In particular, ER diagrams are frequently used during the design stage of a development process in order to identify different system elements and their relationships with each other. For example, an inventory software used in a retail shop will have a database that monitors elements such as purchases, item, item type, item source and item price.
History of ER Diagrams:-
ER diagrams are visual tools that are used in the Entity-Relationship model initially proposed by Peter Chen in 1976 to create a uniform convention that considers both relational database and network views. Chen envisioned the ER model as a conceptual modeling approach that views real world data as systems of entities and relationships. Entities are data objects that maintain different relationships with each other. Additionally, entities are also described further using attributes.
Since 1976, the ER model has been expanded and is sometimes used in business management, product development, and strategy formulations. However, database design remains its primary application.
Elements in ER diagrams :-
There are three basic elements in an ER Diagram: entity, attribute, relationship. There are more elements which are based on the main elements. They are weak entity, multivalued attribute, derived attribute, weak relationship and recursive relationship. Cardinality and ordinality are two other notations used in ER diagrams to further define relationships.
Entity :-
An entity can be a person, place, event, or object that is relevant to a given system. For example, a school system may include students, teachers, major courses, subjects, fees, and other items. Entities are represented in ER diagrams by a rectangle and named using singular nouns.
Weak Entity :-
A weak entity is an entity that depends on the existence of another entity. In more technical terms it can defined as an entity that cannot be identified by its own attributes. It uses a foreign key combined with its attributed to form the primary key. An entity like order item is a good example for this. The order item will be meaningless without an order so it depends on the existence of order.
Attribute :-
An attribute is a property, trait, or characteristic of an entity, relationship, or another attribute. For example, the attribute Inventory Item Name is an attribute of the entity Inventory Item. An entity can have as many attributes as necessary. Meanwhile, attributes can also have their own specific attributes. For example, the attribute “customer address” can have the attributes number, street, city, and state. These are called composite attributes. Note that some top level ER diagrams do not show attributes for the sake of simplicity. In those that do, however, attributes are represented by oval shapes.
Attributes in ER diagrams, note that an attribute can have its own attributes ( composite attribute )
Multivalued Attribute :-
If an attribute can have more than one value it is called an multivalued attribute. It is important to note that this is different to an attribute having its own attributes. For example a teacher entity can have multiple subject values.
Derived Attribute :-
An attribute based on another attribute. This is found rarely in ER diagrams. For example for a circle the area can be derived from the radius.
Relationship :-
A relationship describes how entities interact. For example, the entity “carpenter” may be related to the entity “table” by the relationship “builds” or “makes”. Relationships are represented by diamond shapes and are labeled using verbs.
Recursive Relationship :-
If the same entity participates more than once in a relationship it is known as a recursive relationship. In the below example an employee can be a supervisor and be supervised, so there is a recursive relationship.
Cardinality and Ordinality :-
These two further defines relationships between entities by placing the relationship in the context of numbers. In an email system, for example, one account can have multiple contacts. The relationship in this case follows a “one to many” model. There are number of notations used to present cardinality in ER diagrams. Chen, UML, Crow’s foot, Bachman are some of the popular notations. Creately supports Chen, UML and Crow’s foot notations.
Benefits of ER diagrams : -ER diagrams constitute a very useful framework for creating and manipulating databases. First, ER diagrams are easy to understand and do not require a person to undergo extensive training to be able to work with it efficiently and accurately. This means that designers can use ER diagrams to easily communicate with developers, customers, and end users, regardless of their IT proficiency. Second, ER diagrams are readily translatable into relational tables which can be used to quickly build databases. In addition, ER diagrams can directly be used by database developers as the blueprint for implementing data in specific software applications. Lastly, ER diagrams may be applied in other contexts such as describing the different relationships and operations within an organization.
How to Draw ER Diagrams :-
Because ER diagrams are simple enough to understand, just about anyone can create them. However, two different ER diagrams describing the same system may still be radically different in terms of their simplicity, completeness, and efficiency at communicating the system. In other words, there are good ER diagrams and there are poor ones.
Because this ER tutorial focuses on beginners below are some tips that will help you build effective ER diagrams:
1. Identify all the relevant entities in a given system and determine the relationships among these entities.
2. An entity should appear only once in a particular diagram.
3. Provide a precise and appropriate name for each entity, attribute, and relationship in the diagram. Terms that are simple and familiar always beats vague, technical-sounding words. In naming entities, remember to use singular nouns. However, adjectives may be used to distinguish entities belonging to the same class (part-time employee and full time employee, for example). Meanwhile attribute names must be meaningful, unique, system-independent, and easily understandable.
4. Remove vague, redundant or unnecessary relationships between entities.
5. Never connect a relationship to another relationship.
6. Make effective use of colors. You can use colors to classify similar entities or to highlight key areas in your diagrams.
How to Create an Entity Relationship Diagram
Here are some best practice tips for constructing an ERD:
Identify the entities. The first step in making an ERD is to identify all of the entities you will use.
An entity is nothing more than a rectangle with a description of something that your system stores information about.
This could be a customer, a manager, an invoice, a schedule, etc. Draw a rectangle for each entity you can think of on your page.
Keep them spaced out a bit.
ERD entities
Identify relationships. Look at two entities, are they related? If so draw a solid line connecting the two entities.
Describe the relationship. How are the entities related? Draw an action diamond between the two entities on the line you just added.
In the diamond write a brief description of how they are related.
Add attributes. Any key attributes of entities should be added using oval-shaped symbols.
Complete the diagram. Continue to connect the entities with lines, and adding diamonds to describe each relationship
until all relationships have been described. Each of your entities may not have any relationships, some may have multiple relationships.
