What is an Entity Relationship Diagram (ERD) in Database?

What is an Entity Relationship Diagram (ERD) in Database?

An entity relationship diagram (ERD) is a type of diagram that shows how different entities (such as people, objects, or concepts) relate to each other in a database. An ERD can help you to design, understand, and document the logical structure of a database. In this article, you will learn what an ERD is, what are its components, how to create one, and some tips for making it effective.

Components of an ERD

An ERD consists of the following components:

• Entities: An entity is a thing that can have data stored about it. It can be a physical object (such as a car or a person), a concept (such as an address or a course), or an event (such as a student enrollment or a purchase). An entity is usually represented by a rectangle with the entity name inside it. For example, Student and Course are two entities in a database.

• Attributes: An attribute is a property or characteristic of an entity that can be used to describe it. For example, name, age, and email are attributes of the Student entity. An attribute is usually represented by an oval with the attribute name inside it. Some attributes can be further classified into:

  • Key attributes: A key attribute is an attribute that uniquely identifies an entity. For example, student_id is a key attribute of the Student entity. A key attribute is usually underlined in an ERD.

  • Composite attributes: A composite attribute is an attribute that consists of two or more sub-attributes. For example, address is a composite attribute of the Student entity that can be divided into street, city, state, and zip code. A composite attribute is usually represented by an oval that contains other ovals.

  • Multivalued attributes: A multivalued attribute is an attribute that can have more than one value for an entity. For example, phone_number is a multivalued attribute of the Student entity, as a student can have more than one phone number. A multivalued attribute is usually represented by a double oval.

  • Derived attributes: A derived attribute is an attribute that can be calculated or derived from other attributes. For example, age is a derived attribute of the Student entity, as it can be calculated from the date_of_birth attribute. A derived attribute is usually represented by a dashed oval.

• Relationships: A relationship is a connection or association between two or more entities. For example, enrolls is a relationship between Student and Course entities, as a student can enroll in one or more courses, and a course can have one or more students enrolled in it. A relationship is usually represented by a diamond with the relationship name inside it. There are different types of relationships that can be represented in an ERD:

  • One-to-one (1:1): A one-to-one relationship means that one entity can be related to only one other entity, and vice versa. For example, married_to is a one-to-one relationship between Person entities, as a person can be married to only one other person at a time. A one-to-one relationship is usually represented by a single line between the entities.

  • One-to-many (1:N): A one-to-many relationship means that one entity can be related to many other entities, but each of those entities can be related to only one entity of the first type. For example, teaches is a one-to-many relationship between Teacher and Course entities, as a teacher can teach many courses, but each course can be taught by only one teacher. A one-to-many relationship is usually represented by a line with a crow’s foot at the end near the many entity.

  • Many-to-many (M:N): A many-to-many relationship means that many entities of one type can be related to many entities of another type. For example, enrolls is a many-to-many relationship between Student and Course entities, as a student can enroll in many courses, and a course can have many students enrolled in it. A many-to-many relationship is usually represented by a line with crow’s feet at both ends.

What is Cardinality?

Cardinality is a term that refers to the number of instances or occurrences of an entity that can be associated with another entity in a relationship. Cardinality can be expressed in terms of minimum and maximum values. For example, in the enrolls relationship between Student and Course entities, the cardinality can be expressed as:

• Each Student must enroll in at least one Course (minimum cardinality = 1).
• Each Student can enroll in at most four Courses (maximum cardinality = 4).
• Each Course must have at least five Students enrolled in it (minimum cardinality = 5).
• Each Course can have at most 30 Students enrolled in it (maximum cardinality = 30).

Cardinality can also be expressed using natural language phrases such as “one and only one”, “one or more”, “zero or more”, “zero or one”, etc. For example, in the married_to relationship between Person entities, the cardinality can be expressed as:

• Each Person can be married to zero or one Person (cardinality = 0…1).
• Each Person is married to one and only one Person (cardinality = 1).

Cardinality can be shown on an ERD using different notations, such as numbers, symbols, or labels. For example, the following ERD shows the cardinality of the enrolls relationship using numbers:

![ERD with cardinality]

How to Create an Entity Relationship Diagram (ERD)

To create an entity relationship diagram, you need to follow these steps:

• Identify the entities and attributes of your database. You can use nouns and adjectives to name them.

• Identify the relationships and cardinalities between the entities. You can use verbs and phrases to name them.

• Draw the ERD using the symbols and notations for entities, attributes, and relationships. You can use tools such as Visual Paradigm¹, Guru99², or Database Star³ to create your ERD online or offline.

• Check the validity and completeness of your ERD. You can use questions such as:

  • Does your ERD capture all the data requirements of your database?
  • Does your ERD avoid any redundancy or inconsistency in your data?
  • Does your ERD follow the rules of normalization and integrity constraints?
  • Does your ERD use clear and consistent naming conventions and formats?

Tips for Creating an Entity Relationship Diagram

Here are some tips for creating an effective entity relationship diagram:

• Use meaningful and descriptive names for your entities, attributes, and relationships. Avoid using abbreviations or acronyms that may cause confusion or ambiguity.
• Use singular nouns for your entities and attributes. For example, use Student instead of Students, and name instead of names.
• Use proper capitalization and punctuation for your names. For example, use Course_ID instead of course_id or course id.
• Use consistent levels of abstraction for your entities and attributes. For example, do not mix general and specific entities or attributes in the same ERD.
• Use appropriate data types and formats for your attributes. For example, use numeric data types for numbers, date data types for dates, etc.
• Avoid using unnecessary or redundant entities, attributes, or relationships. For example, do not include an attribute that can be derived from another attribute, or a relationship that can be inferred from another relationship.
• Use comments or notes to explain any complex or unclear aspects of your ERD. For example, you can use comments to define the meaning or scope of an entity, attribute, or relationship.

Conclusion

An entity relationship diagram (ERD) is a useful tool for designing, understanding, and documenting the logical structure of a database. An ERD consists of entities, attributes, and relationships that represent the data requirements of a database. An ERD also shows the cardinality of the relationships between the entities. To create an ERD, you need to identify the entities, attributes, relationships, and cardinalities of your database, and draw them using symbols and notations. You also need to check the validity and completeness of your ERD, and follow some tips for making it effective.

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