How to Implement Multiple One-to-One Relationships in a Relational Database?

Creating Multiple One-to-One Relationships in Relational Databases

One common challenge in database design is establishing relationships between tables to represent real-world scenarios. In this case, the task is to create multiple one-to-one relationships between the Storage table and the Van and Warehouse tables.

To understand this issue, let's examine the provided database schema:

Inventory Table <*-----1> Storage Table <1-----1> Van Table
                                  ^
                                  1
                                  |-------1> Warehouse Table

The Storage table is utilized because the Van and Warehouse tables have similarities. However, creating a direct relationship between the Storage table and both the Van and Warehouse tables while ensuring one-to-one correspondence can be tricky.

There are several ways to address this issue:

1. All Classes in One Table:

This approach involves creating a single table that includes columns for the parent (Storage) and all child (Van and Warehouse) classes. Constraints are then applied to ensure that specific fields are non-NULL for each child class. This method is often considered less desirable due to potential database complexity.

2. Concrete Class per Table:

With this approach, separate tables are created for each child class without a dedicated parent table. While this simplifies the database structure, relationships between the parent (Inventory) and the children must be repeated in each child table, potentially leading to data duplication.

3. Class per Table:

This is the preferred method and involves creating a parent table (Storage) and separate tables for each child class (Van and Warehouse). While it provides the cleanest database structure, it can introduce some performance trade-offs during data modification operations.

To enforce both the presence and exclusivity of a child in the database, one can utilize deferred constraints. However, Microsoft SQL Server lacks support for this feature. Alternatively, manual enforcement measures can be implemented through stored procedures or application-level logic to manage the relationship between the Storage, Van, and Warehouse tables.

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