One-to-many Database Relationship | Information Systems and Control ICS CPA Exam

In this video, I explain one to many ( 1 to many) relationship. A one-to-many database relationship is a relationship between two database tables where a record in one table can reference several records in another table.

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One-to-Many Relationship (1:M)

A one-to-many relationship in database design is a connection between two tables where a record in the first table (often called the parent or primary table) can have multiple related records in the second table (often referred to as the child or foreign table). However, a record in the second table can be related to only one record in the first table.

How it works:

1. The primary table contains a primary key, which is a unique identifier for its records.
2. The secondary table contains a foreign key column, which holds the primary key value of related records from the primary table.

Example:

1. **Entities**: `Authors` and `Books`

Authors Table:

| AuthorID | AuthorName |
|----------|------------|
| 1 | J.K. Rowling |
| 2 | George Orwell |

**Books Table:**

| BookID | Title | AuthorID |
|--------|----------------------|----------|
| 1 | Harry Potter | 1 |
| 2 | 1984 | 2 |
| 3 | Animal Farm | 2 |
| 4 | The Casual Vacancy | 1 |

- Here, `AuthorID` in the `Authors` table is the primary key.
- `AuthorID` in the `Books` table is the foreign key linking back to the `Authors` table.

From the data:

- J.K. Rowling (from the `Authors` table) has written both "Harry Potter" and "The Casual Vacancy" (two records in the `Books` table).
- George Orwell has written "1984" and "Animal Farm".
- Thus, one author can write many books, establishing a one-to-many relationship.

Why is it Important?

One-to-many relationships are essential in database design as they:

1. Minimize Data Redundancy**: Instead of repeating author details for every book they've written, you just link the book to an author's ID.
2. Ensure Data Integrity**: By using foreign keys, databases ensure that orphan records (like a book without a valid author) cannot exist.
3. Facilitate Efficient Queries**: Fetching related data becomes more streamlined when using relationships. For instance, you can easily retrieve all books written by a particular author by performing a join operation on the two tables.

Understanding one-to-many relationships is fundamental when dealing with relational databases, as they form the backbone of many database designs.