In relational database design, why do you need common fields columns in different tables?

Introduction / Context:
Relational databases are built on the idea of storing related pieces of data in separate tables and then connecting them logically. Common fields, also known as common columns or key columns, are essential to this design. They allow tables to be related to one another in a meaningful way so that queries can bring together data from multiple sources. This question explores why these common fields are necessary in a well structured database.

Given Data / Assumptions:

• The database follows the relational model with tables, rows, and columns.
• Different tables store different aspects of related entities, such as customers and orders.
• Some columns appear in more than one table, for example customer identifiers.
• These common fields are used to relate tables.
• We are choosing the best explanation for their purpose.

Concept / Approach:
Common fields exist primarily to support relationships between tables. A typical pattern is a primary key in one table and a matching foreign key in another. When two tables share a common field, queries can join them based on that field, allowing the database to combine related records into a single result set. This supports normalisation by avoiding redundant data, while still enabling comprehensive queries across related tables.

Step-by-Step Solution:
Step 1: Identify common fields as columns that hold the same type of data, such as a customer identifier, in multiple tables. Step 2: Recognise that these fields are often primary keys in one table and foreign keys in another. Step 3: Understand that joins in SQL use these common values to connect rows from different tables. Step 4: Evaluate the options and select the one that clearly states that common fields exist to create logical relationships and enable joins. Step 5: Confirm that this explanation fits standard relational database design principles.

Verification / Alternative check:
If you consider classic examples like a Customers table with CustomerId and an Orders table that includes CustomerId as a foreign key, you can see that the common field allows you to join orders back to their customers. Without this shared column, you could not easily link data across tables, defeating the purpose of the relational model. This example validates that relationships and joins are the key reasons for common fields.

Why Other Options Are Wrong:
Option B is incorrect because the goal of good database design is to reduce unnecessary duplication, not to store duplicate data for speed. Option C is wrong since common fields do not reduce the need for indexes; they often require indexes to support efficient joins. Option D is the opposite of relational design, which encourages using tables together rather than in isolation. Option E is incorrect because common fields usually support foreign keys and constraints instead of preventing them.

Common Pitfalls:
A common misunderstanding is thinking that common fields are just repeated columns with no structural purpose. Another pitfall is assuming that duplication of data is the primary goal, when in fact normalisation aims to minimise redundancy while preserving the ability to relate data logically. Always think of common fields as the glue that connects tables through keys and joins.

Final Answer:
You need common fields in a database to create logical relationships between tables and enable joins that combine related data..