How to Effectively Map Object-Oriented Inheritance to a Relational Database?

How to Map Object-Oriented Inheritance in a Relational Database

Object-oriented programming (OOP) is a popular paradigm that incorporates concepts such as inheritance and polymorphism. However, when it comes to persisting objects in a relational database, the question arises of how to effectively model object-oriented inheritance within a relational database schema.

Martin Fowler, in his book "Patterns of Enterprise Application Architecture," provides a detailed discussion on this topic. He proposes three primary approaches for mapping inheritance in a relational database:

1. Single Table Inheritance:

This approach stores all classes in a single table. A column is added to the table to identify the type of object. This method is simple and easy to implement, but it can lead to a table with many columns and null values.

2. Class Table Inheritance:

In this approach, a separate table is created for each class. Each table shares a common parent table that stores the common attributes. This method reduces the number of columns in each table, but it requires more tables and can make queries more complex.

3. Concrete Table Inheritance:

Concrete Table Inheritance is an extension of Class Table Inheritance. A separate table is created for each subclass, and the shared attributes are stored in the tables of the lowest-level classes. This method provides the most flexibility but can result in a complex schema.

The optimal choice between these approaches depends on the specific requirements and trade-offs involved in the application. Single Table Inheritance may be suitable for simple inheritance scenarios with few shared attributes, while Class Table Inheritance or Concrete Table Inheritance may be more appropriate for complex inheritance hierarchies with numerous shared attributes.

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