# Cardinality in ER Diagram

## Introduction

When designing a database, it's crucial to understand the relationships between different entities. Entity-relationship (ER) diagrams represent the relationships between entities in a database. However, more is needed to connect entities in a diagram - we also need to define the nature of those relationships, and that's where the concept of cardinality comes in.

Cardinality refers to the number of instances of one entity related to instances of another entity. In other words, it's a way of quantifying the relationship between entities in an ER diagram. By defining the cardinality of a relationship, we can establish important rules that govern how data is stored and accessed in a database.

There are several types of cardinality, each representing a different type of relationship between entities. These include one-to-one, one-to-many, many-to-one, and many-to-many relationships. We can create more accurate and effective database designs by understanding the different types of cardinalities and how they are represented in ER diagrams.

However, cardinality is only sometimes straightforward. There are often complex scenarios in which multiple relationships between entities exist or where entities relate to themselves in recursive relationships. In these cases, understanding cardinality becomes even more important to create a well-designed database that meets users' needs.

In this article, we'll explore the concept of cardinality in more detail, including the different types of cardinality, how they are represented in ER diagrams, and how to apply cardinality to common scenarios in database design. By the end of this article, you'll better understand how to use cardinality to design accurate and efficient databases effectively.

## What is Cardinality and its Types?

Cardinality is a term used in database design for the relationship between two tables. Understanding cardinality when designing a database is important because it can help ensure data is stored and accessed efficiently. Cardinality defines the number of instances of one entity associated with another in a database.

There are four main types of cardinality: one-to-one (1:1), one-to-many (1:N), many-to-one (N:1), and many-to-many (N: N). Each of these Cardinality types has its advantages and disadvantages, and the type of cardinality you choose will depend on the specific needs of your database.

### 1. One-to-One Cardinality

One-to-one cardinality occurs when each instance of one entity is associated with only one instance of another and vice versa. This type of cardinality is rare in databases because combining the two entities into a single table is usually more efficient. However, there are some cases where one-to-one relationships are useful. For example, if you have a database for a hospital, each patient might be associated with a single bed in a hospital room.

### 2. One-to-Many Cardinality

One-to-many cardinality occurs when each instance of one entity is associated with multiple instances of another. Still, each instance of the second entity is, and there is, only one instance of the first entity associated with it. This type of cardinality is the most common type of relationship in databases. For example, if you have a database for a library, each author might have written many books, but each book can only have one author.

### 3. Many-to-One Cardinality

Many-to-one cardinality is the inverse of one-to-many cardinality. It occurs when multiple instances of one entity are associated with a single instance of another entity. For example, if you have a database for a school, many students might be associated with a single teacher. Many-to-one relationships are less common than one-to-many relationships, but they can be useful sometimes.

### 4. Many-to-Many Cardinality

Many-to-many cardinality occurs when multiple instances of one entity are associated with multiple instances of another. This type of cardinality is more complex than one-to-many or many-to-one relationships, requiring an additional table to manage the relationship. For example, if you have a database for a music streaming service, many users might have many playlists, and each playlist might contain many songs.

## Managing Cardinality

Cardinality is managed by using foreign keys. A foreign key is a field in a table that refers to the primary key of another table. For example, if you have a database for a library, the book table might have a foreign key that refers to the author table. This foreign key is used to establish the relationship between the two tables.

When managing cardinality, ensuring that data is not duplicated unnecessarily is important. For example, in a one-to-many relationship, we can ensure that data is not duplicated by storing the foreign key of the "many" entity in the "one" entity. It ensures that each instance of the "one" entity is associated with only one instance of the "many" entity.

## Cardinality and Database Design

Cardinality is an important concept in database design because it helps ensure data is stored and accessed efficiently. Choosing the right type of cardinality is critical to creating an efficient and effective database.

One of the key advantages of using cardinality in database design is that it helps ensure that data is consistent. For example, if you have a database for a library, using one-to-many cardinality to associate each book with a single author helps ensure that each book has a valid author.

Another advantage of using cardinality in database design is that it can help reduce the amount of data that needs to be stored in a database. By establishing relationships between tables, we can avoid storing duplicate data. For example, in a one-to-many relationship, we can store the foreign key of the "many" entity in the "one" entity rather than storing the entire "many" entity in each instance of the "one" entity. It can result in significant storage savings.

Cardinality also helps ensure data integrity. By defining the relationships between tables, we can enforce constraints on the data entered into the database. For example, in a one-to-many relationship between a customer table and an order table, we can ensure that each order is associated with a valid customer.

When designing a database, it is important to consider the cardinality of each relationship carefully. Cardinality will impact the database's structure, the queries' performance, and the data storage's efficiency. It is also important to consider the potential for future changes to the database. For example, suppose you initially choose a one-to-many relationship between two tables but later realize that a many-to-many relationship would be more appropriate. In that case, you may need to modify the database structure.

Finally, cardinality is an important concept in database design that refers to the relationship between two tables. There are four main types of cardinality: one-to-one, one-to-many, many-to-one, and many-to-many. Each type of cardinality has its advantages and disadvantages, and the type of cardinality you choose will depend on the specific needs of your database.

Cardinality is managed using foreign keys, which establish the table's relationship. By carefully considering the cardinality of each relationship, you can ensure that your database is efficient and effective and maintains data integrity.

## Examples of Cardinality in ER Diagrams

Cardinality is an essential concept in entity-relationship (ER) diagrams, which is used to depict the relationships between entities in a database. Cardinality in ER diagrams determines the number of entities or records in one table associated with the records in another.

Various symbols in ER diagrams usually represent cardinality, indicating the nature of the relationships between entities. This section will discuss some examples of Cardinality in ER diagrams.

### 1. One-to-One Cardinality

One-to-one cardinality is a relationship between two entities where one entity can be related to only one instance of another entity. In this relationship, each record in one table has a corresponding record in another table. This type of relationship is rare, but it can be useful in certain situations.

For example, consider the relationship between a person and their passport. Each person can have only one passport associated with only one person, and it is a one-to-one relationship, as shown in the following ER diagram:

``````   Person     Passport
------     --------
PK_ID       PK_ID
Name        Number
DOB         Expiration_Date
Passport_ID FK_Person_ID

``````

In this example, the foreign key FK_Person_ID in the Passport table refers to the primary key PK_ID in the Person table.

### 2. One-to-Many Cardinality

One-to-many cardinality is the most common relationship in ER diagrams. In this relationship, one record in one table can be associated with many records in another. Still, each record in the second table can be associated with only one record in the first table.

For example, consider the relationship between a customer and their orders. A customer can have multiple orders, but each order is associated with only one customer. It is a one-to-many relationship, as shown in the following ER diagram:

``````+---------------+               +---------------+
|   Customer   |                |   Order   	 |
+---------------+               +---------------+
| customer_id |                 | order_id       |
|   name          |       	|   date         |
|  address        |       	|  amount   	 |
|   phone         |       	+--------------+
+---------------+                |customer_id|
+-------------+

``````

In the above diagram, the "Customer" entity is associated with multiple instances of the "Order" entity. However, each instance of the "Order" entity is associated with only one instance of the "Customer" entity.

The symbols near the entities indicate the cardinality of the relationship. The "Customer" entity has a cardinality of one, represented by the symbol (1), while the "Order" entity has a cardinality of many, represented by the symbol (N).

The foreign key "customer_id" in the "Order" entity further defines the relationship. This key establishes a connection between the "Customer" and the "Order" entities, ensuring that each order is associated with only one customer.

### 3. Many-to-One Cardinality:

Many-to-one cardinality is a relationship between two entities where many instances of an entity can be related to one instance of another entity. Still, each instance of the first entity can have only one associated entity instance.

An example of many-to-one cardinality is the relationship between "Order" and "Product" entities in an online store's ER diagram. Each order can include many products, but each can be included in only one order. The relationship's cardinality is many-to-one, indicated by the symbol (N:1).

Here is an ER diagram representing the many-to-one cardinality relationship:

``````+-------------+                  +-------------+
|   Order     |                  |   Product   |
+-------------+                  +-------------+
| order_id    |--(N:1)----------| product_id  |
|  date       |                 |   name      |
|  customer   |                 |  price      |
|  status     |                 |  quantity  |
+-------------+                 |   vendor   |
+-------------+
| order_id   |
+-------------+

``````

In the above diagram, the "Order" entity has a cardinality of many, represented by the symbol (N), while the "Product" entity has a cardinality of one, represented by the symbol (1). It indicates that each order can include many products but can be included in only one order.

The foreign key "order_id" in the "Product" entity further defines the relationship. This key establishes a connection between the "Order" and the "Product" entities, ensuring that each product is associated with only one order.

### 4. Many-to-Many Cardinality:

Many-to-many cardinality is a relationship between two entities where many instances of an entity can be related to many instances of another entity and vice versa. This type of relationship requires using a junction or associative entity to manage the relationship.

An example of many-to-many cardinality is the relationship between the "Student" and "Course" entities in a university's ER diagram. Each student can take many courses, which many students can take. The relationship's cardinality is many-to-many, indicated by the symbol (N: M).

Here is an ER diagram representing the many-to-many cardinality relationship:

``````+------------+            +------------+             +-----------------+
|   Student   |           |   Course   |             | Enrollment     |
+------------+            +------------+             +-----------------+
| student_id |--(N:M)-----| course_id |              |enrollment_id |
|   name      |           |   title    |             |  student_id    |
|  address    |           |  credits   |             |  course_id     |
+------------+            |  dept_id  -|--(1:N)--+   date            |
+------------+             +----------------+

``````

In the above diagram, the "Enrollment" entity acts as a junction entity between the "Student" and "Course" entities, managing the many-to-many relationship between them.

Understanding the cardinality of relationships between entities is essential in designing an effective database. Properly defining and representing cardinality in an ER diagram helps ensure data integrity and consistency and enables efficient querying and retrieval of data.

## Conclusion

In conclusion, cardinality play a critical role in entity-relationship (ER) diagrams by defining the relationships between entities and their minimum and maximum occurrences. Cardinality is represented using symbols such as (1:1), (1:N), (N:1), and (N: N), which represent one-to-one, one-to-many, many-to-one, and many-to-many relationships, respectively.

Accurately defining cardinality constraints is essential in developing efficient and effective database management systems, as improperly defined constraints can lead to data redundancy, inconsistencies, and errors. Cardinality constraints help understand the relationships between entities and facilitate the development of data models that accurately represent real-world situations.

ER diagrams are a crucial component of database design and development, and the correct identification and definition of cardinality constraints are crucial for their effectiveness. Using cardinality constraints helps reduce data duplication, improve data accuracy, and facilitate data retrieval and analysis.