Understanding Inheritance in Database Concepts in SQL Server 2005

When designing a database, you might come across the concept of inheritance, which is frequently used in programming to derive new classes from existing ones, allowing for shared properties and methods. However, when working with SQL Server 2005, many users wonder if they can use similar principles of inheritance within their database tables. Specifically, the challenge often involves including common fields—like CreatedOn and CreatedBy—across multiple entities without manually repeating these fields in every table.

In this post, we will explore the limitations of inheritance in SQL Server 2005 and alternative solutions you can adopt to manage shared data efficiently.

The Inheritance Challenge in SQL Server 2005

To start, SQL Server 2005 does not natively support inheritance between tables in the manner you might expect from object-oriented programming. This means that you cannot directly create a “base” table that other tables inherit from, where they automatically receive the schema (fields/columns) from this parent table.

Why Inheritance Doesn’t Exist

• Table Structure: Each table in SQL Server is independent. While you can create relationships between them using foreign keys, the concept of inheriting columns automatically from one table to another doesn’t apply in traditional SQL database design.
• Common Use Cases: Many users think of inheritance as a way to streamline their data models, particularly when repeated fields need to be ubiquitous in multiple entities (like audit fields).

Solutions to Manage Shared Fields

Even though true inheritance isn’t an option, there are ways to effectively manage commonly shared fields across different tables. Here are a couple of approaches you can consider:

1. Using a Shared Table with Foreign Keys

One method to create a more organized structure is by using a separate table dedicated to common fields. For instance:

• Create a Shared Table: Create a table that includes CreatedOn, CreatedBy, and other commonly shared fields.

  CREATE TABLE SharedMetadata (
      ID INT PRIMARY KEY,
      CreatedOn DATETIME,
      CreatedBy VARCHAR(100)
  );
  

• Link with Foreign Keys: Link this shared metadata table to other entity tables as needed. Each entity table can reference the ID of the SharedMetadata table through a foreign key.

  CREATE TABLE EntityA (
      ID INT PRIMARY KEY,
      MetadataID INT,
      FOREIGN KEY (MetadataID) REFERENCES SharedMetadata(ID)
  );
  

Pros:

• Maintains a single record of common fields.
• Reduces redundancy and potential inconsistency.

Cons:

• Requires additional joins when accessing the common fields.
• Involves managing relationships, which may add complexity.

2. Manual Addition of Common Fields

If the table structure isn’t overly complicated, manually adding the common fields might be straightforward for smaller applications or projects.

• You would simply declare the fields CreatedOn and CreatedBy in each table:

  CREATE TABLE EntityA (
      ID INT PRIMARY KEY,
      CreatedOn DATETIME,
      CreatedBy VARCHAR(100)
  );
  
  CREATE TABLE EntityB (
      ID INT PRIMARY KEY,
      CreatedOn DATETIME,
      CreatedBy VARCHAR(100)
  );
  

Pros:

• Simplicity in table design.
• No complex relationships to manage.

Cons:

• Redundancy of data across multiple tables.
• Higher possibility of inconsistencies, as updates to field values must be repeated in multiple places.

Conclusion: Manual Effort Required

In summary, while SQL Server 2005 lacks a direct method for implementing inheritance like in programming languages, you can adopt effective strategies such as creating a shared metadata table or repeating commonly used fields across your tables. However, both methods come with their trade-offs, and ultimately, some level of manual work will be involved to maintain your database structure. Always consider your application’s needs and choose the solution that best fits your design requirements.

By understanding these limitations and exploring viable alternatives, you will be better equipped to manage your data efficiently within SQL Server 2005.