Using Inheritance Mapping Strategies (Single Table, Joined, Table Per Class)

When designing a database schema for an application, one common requirement is to represent a hierarchy of objects with relationships and properties. In object-oriented programming, inheritance is a powerful concept that allows us to organize and reuse code by defining a base class and extending it with more specific subclasses. However, mapping this hierarchy to a relational database poses some challenges. Hibernate and JPA provide three inheritance mapping strategies: single table, joined, and table per class. Let's explore each of these strategies in detail.

Single Table Strategy

The single table strategy, also known as the superclass table strategy, represents the entire class hierarchy in a single database table. All properties of the entire hierarchy are mapped to the columns of this table. Additionally, a discriminator column is used to determine the specific subclass for each row.

This strategy has several advantages:

It is simple to implement and understand, as it requires only one table.
It provides decent performance, especially for fetching objects of the base class or querying objects that share common properties.
It does not require any joins, resulting in faster query execution.

However, it also has some drawbacks:

The table can become crowded when the hierarchy becomes large, leading to decreased performance.
Nullable columns are common since not all properties will be applicable to all subclasses, resulting in wasted space.
Changing the superclass table structure could potentially impact all subclasses.

Joined Strategy

The joined strategy, also known as the subclass table per concrete class strategy, represents each class in the hierarchy with its own dedicated table. The common properties of the superclass are stored in a separate table, with a foreign key relationship to each subclass table. This strategy allows each subclass table to only include its unique properties.

This strategy has the following advantages:

It offers a well-structured database schema, avoiding duplicate columns and providing better data integrity.
It supports nullability as columns are specific to each subclass, reducing wasted space.
It allows for efficient querying by providing a clear separation of tables.

However, it also has some limitations:

Performing queries requiring properties from multiple subclasses involves using joins, which can impact performance.
Modifying the superclass table structure does not impact subclasses, but modifying the structure of a subclass table affects only that particular class.

Table per Class Strategy

The table per class strategy, also known as the concrete table per class strategy, represents each class in the hierarchy with its own dedicated table. Unlike the joined strategy, there is no common table for superclass properties. Instead, each table contains all properties associated with its respective class.

This strategy has the following advantages:

It offers a clean and independent schema for each class, allowing for better performance and flexibility.
It does not require any joins, resulting in faster query execution.
Modifying the structure of one table does not impact other tables.

However, it also has some drawbacks:

It can lead to redundant columns and a potentially inconsistent database schema.
Referential integrity needs to be maintained manually since there is no inherent relationship between tables representing the same class hierarchy.
Querying objects from the entire hierarchy can be complex and inefficient.

Conclusion

Choosing the right inheritance mapping strategy depends on the specific requirements and characteristics of your application. The single table strategy provides simplicity, while the joined and table per class strategies offer better database schema organization. Consider factors such as hierarchy size, performance requirements, and flexibility when deciding on the most suitable strategy.

Hibernate and JPA provide flexible options for mapping inheritance hierarchies, ensuring that you can efficiently represent complex object models while maintaining a well-designed database schema.