Creating Association between Two Entries in a SQL Table

Introduction

In this article, we will explore how to create an association table that links two entries from different tables. This is a common requirement when designing databases for applications that require relationships between data entities.

We will use a real-world example with five tables: Customers, Accounts, Associations, Security (Collateral), and References (Reference Codes relating to a Job type). Our goal is to create an Association table that links two customers based on their association type.

Table Structure

Before we dive into the details of creating the Association table, let’s review the structure of our existing tables:

Customers Table

Accounts Table

Associations Table

Security Table

References Table

Foreign Keys

In the Associations Table, we have two foreign keys: Customer1 and Customer2. These foreign keys reference the primary key of the Customers table.

To establish this relationship, we need to create foreign key constraints on these columns:

alter table associations add constraint fk_associations_customer1 
    foreign key (customer1_id) references customers (customer_id);

alter table associations add constraint fk_associations_customer2 
    foreign key (customer2_id) references customers (customer_id);

In the above code snippet, we are creating two foreign key constraints: fk_associations_customer1 and fk_associations_customer2. These constraints ensure that the values in the Customer1 and Customer2 columns match the primary keys of the Customers table.

Defining the Primary Key

Before we can create these foreign key constraints, we need to define the primary key on the Customers table. The primary key is a unique identifier for each record in the table. In this case, let’s assume that customer_id is the primary key:

create table customers (
    customer_id int primary key, 
    name varchar(255),
    email varchar(255)
);

In the above code snippet, we are creating a new table called customers. The customer_id column is defined as the primary key using the primary key constraint.

Naming Conventions

When designing tables and relationships, it’s essential to follow standard naming conventions:

• Tables are typically in the plural form (e.g., customers, accounts, etc.).
• The primary key is usually named as the singular form followed by _id (e.g., customer_id, account_id, etc.).
• Foreign keys often have the same name as or very similar to their corresponding primary keys.

Conclusion

In this article, we explored how to create an Association table that links two entries from different tables using foreign key constraints. We also reviewed common naming conventions for tables and relationships in SQL database design.

By following these best practices and understanding the intricacies of foreign key relationships, you can create efficient and scalable databases for your applications.

Table of Contents

• Creating Association between Two Entries in a SQL Table
  • Table Structure
    • Customers Table
      • Columns
        1. customer_id
        2. name
        3. email
    • Accounts Table
      • Columns
        1. account_id
        2. account_type
        3. customer_id
    • Associations Table
      • Columns
        1. AssociationID
        2. Customer1
        3. AssociationType
        4. Customer2
    • Security Table
      • Columns
        1. security_id
        2. customer_id
    • References Table
      • Columns
        1. reference_id
        2. job_type
        3. customer_id
  • Foreign Keys
    • Creating Foreign Key Constraints
  • Defining the Primary Key
    • Defining the Primary Key on the Customers Table
  • Naming Conventions
    • Tables in the Plural Form
    • Singularized Primary Keys with _id
    • Similar Foreign Key Names

Last modified on 2023-06-09