A diagram called an entity relationship shows the relationships between Entities in a database. It is frequently referred to as an ER Diagram. In DBMS, an ER Diagram is essential for constructing the database. In the commercial world of today, an ER Diagram serves as a preview of all user requirements. The database administrators will use it to design the database.
What is an ER Diagram?
The relationship between entities that will be recorded in a database is illustrated in an entity relationship diagram (ER Diagram). The ER Diagram is essentially a database’s structural design. It serves as a framework built with specific symbols to define the relationships between database entities. Entities, attributes, and relationships make up the three main parts of an ER diagram.
The relationship between the two entities Student and Course is shown in the above diagram. As a course may be chosen by many students and a student can choose more than one course, the relationship between student and course is stated as being many-to-many. Student entity possesses attributes – Stu_Id, Stu_Name & Stu_Age. The course entity has attributes such as Cou_ID & Cou_Name.
An Entity-Relationship Model uses a diagram to illustrate the database’s structure. It takes a methodical approach to create a database since, before creating your database, you must assess all the data needs.
History of ER Models in DBMS
In order to provide a standardized convention that may be utilized as a conceptual modeling tool, Peter Chen created ER Diagrams in 1971. There were several models offered and considered, but none were appropriate. His idea was also influenced by Charles Bachman’s data structure diagrams.
Why Use ER Diagrams in DBMS?
Reasons to use ER diagram in DBMS are:
- You may visualise the database and determine which fields must be embedded for a certain entity using the ER Diagram.
- A better understanding of the data to be kept in a database is provided by an ER Diagram.
- It simplifies things and makes it possible for database designers to create databases rapidly.
- Entity-Relationship models are useful for providing descriptions of items.
- It enables users to obtain a glimpse of the database’s logical structure.
Symbols Used in ER Diagrams
Below are the symbols that are used in ER Diagrams:
- Rectangles: This entity-type symbol is used in entity relationship diagrams.
- Ellipses: This symbol represents attributes.
- Diamonds: This symbol represents relationship types.
- Lines: It connects entities with other relationship types and with attributes.
- Primary key: Here, it underlines the attributes
- Double Ellipses: It represents multi-valued attributes
Components of ER Diagram
An ER Diagram is based on three fundamental ideas:
- Weak Entity
- Key Attribute
- Composite Attribute
- Multivalued Attribute
- Derived Attribute
- One-to-One Relationships
- One-to-Many Relationships
- Many-to-One Relationships
- Many-to-Many Relationships
An entity can be a living being or a non-living thing. In an ER diagram, it displays an entity as a rectangle. For instance, the student and the course are both entities in a student study course.
- Weak Entity
A weak entity is one that relies on another entity for its existence.
In the ER Diagram, you display the weak entity as a double rectangle.
The fact that the school in the example below has the main key property "school number" makes it a strong entity. The classroom, unlike the school, is a weak entity as it lacks a primary key and the room number here merely serves as a discriminator.
An attribute displays an entity’s characteristics. In an ER diagram, an oval shape can be used to represent an attribute.
Types of Attributes:
There are four types of attributes that are discussed below:
- Key Attribute
An entity from a group of entities is uniquely identified by its key characteristic. It underlines a crucial attribute’s text in the text.
The roll number, for instance, can be used to distinguish one student from another in a group of students.
- Composite Attribute
A composite characteristic is one that is made up of many other attributes.
The composite attribute is represented by an oval, which is further related to additional ovals.
- Multivalued Attribute
Multivalued attributes are those where an attribute can have many values.
A multivalued attribute is represented by a double oval shape.
- Derived Attribute
A derived attribute is one that may be derived from the entity’s other characteristics.
The derived attribute is represented by the dashed oval in the ER diagram.
In the ER diagram, a relationship can be seen in the diamond shape.
It demonstrates the connections between the two items.
In the scenario below, the student and the course are viewed as different entities, and study is the interaction between them.
Types of Relationships
Mainly we have four types of relationship in ER model:
- One-to-One Relationship
A one-to-one connection exists when a single element of one entity is connected to a single element of another entity.
For instance, a student only has one ID, and only one individual is allowed to have an ID.
- One-to-Many Relationship
A one-to-many connection exists when a single element of one entity is connected to several elements of another entity.
For instance, a consumer can place several orders, but many customers cannot place the same order.
- Many-to-One Relationship
A many-to-one connection is one in which more than one element of one entity is connected to only one element of another.
For instance, although only one course is available, students may enrol in many courses.
- Many-to-Many Relationship
A many-to-many connection exists when several elements of one entity relate to multiple elements of another entity.
An employee, for instance, may be assigned to several projects, and a project may have several employees.
How to Draw an ER Diagram?
Below are some important points to draw ER diagram:
- Identify each Entity first. Put each entity inside a rectangle and give it a descriptive name.
- Establish connections between entities that are related to one another by placing a diamond in the middle to signify the connection. Do not relate relationships to one another.
- Connect entity characteristics and give them the appropriate labels.
- Eliminate any linkages or entities that are unnecessary.
- Make sure your ER Diagram can accommodate all the information used to create the database.
- Use colour to draw attention to important regions in your diagrams.
Features of ER Model
The features of ER Model are as follows −
- ER Diagram: The diagrams that are drawn up to design the database are called ER diagrams. They are built on the basis of three fundamental ideas: entities, characteristics, and connections between them. We specify the entities, their associated properties, and the relationships between them in the ER diagram. This aids in demonstrating the databases’ logical structure.
- Database Design: The Entity-Relationship model supports database designers in creating databases in an extremely straightforward and logical way.
- Graphical Representation helps in Better Understanding: Because ER diagrams are so straightforward to read and comprehend, developers may utilise them with ease to communicate with stakeholders.
- Easy to build: The ER model is fairly simple to construct.
- The extended E-R features: Specialization, upper- and lower-level entity sets, attribute inheritance, aggregation, and generalisation are a few of the other elements of the ER paradigm.
- Integration of ER model: This model is frequently used by database designers to convey their ideas and may be included in a dominating common relational model.
- Simplicity and various applications of ER model: It offers a sneak peek of how all your tables should interact and what fields each table will have, serving as a template for integrating data into certain software programs.
The conceptual design of databases is frequently described by an ER Diagram in DBMS. Viewing the database’s structure before adopting it benefits both users and database developers.
FAQs related to ER Diagram in DBMS
1. What are the 4 main parts of DBMS?
Entities, attributes, and relationships are the three fundamental building blocks of ER diagrams or ERDs.
2. What is the purpose of ER?
In particular, the synthesis, folding, modification, and transport of proteins are all processes carried out by the endoplasmic reticulum (ER).
3. What are the main advantage of ER Model in DBMS?
An E-R model makes use of simple illustrations to make the data needs comprehensible. A relational database may be created quickly from the E-R concept. Because the elements and relationships between them are clearly stated, the E-R diagram is very simple to grasp.
4. What are the limitations of ER?
Only that relationship structure is shown in ER diagrams. Not for unstructured data: ER diagrams are likely to be of little utility unless the data is well segmented into several fields, rows, or columns. Semi-structured data is similar in that only portion of it will be helpful.
5. What is the difference between smooth ER and rought ER?
The presence of ribosomes distinguishes RER from SER in the most fundamental way. An ER is known as Rough ER because of the distinctively rough look it takes on when ribosomes connect to its surface. A smooth ER, on the other hand, doesn’t have ribosomes on its surface. It has ribosomes that are attached to its membrane.