Unified or Separate NOTES Tables: Which Database Design Best Handles Subtypes?

Subtypes: A Dilemma in Database Design

In the realm of database design, the question of subtyping arises when dealing with entities that share common characteristics but also exhibit unique attributes. Consider a scenario where you have three tables: BOOKS, ARTICLES, and NOTES. Each book or article can have multiple notes.

Original Design: A Unified NOTES Table

The initial design opted for a unified NOTES table with the following columns:

• note_id
• note_type
• note_type_id
• note_content

In this schema, note_type indicates whether the note is associated with a book or an article, while note_type_id serves as the foreign key to the respective entity.

Alternative Design: Separate NOTES Tables

An alternative approach suggests splitting NOTES into separate tables:

• notes
• book_notes
• article_notes

This design segregates book-specific and article-specific notes, introducing additional tables for linking to the corresponding entities.

Evaluating the Options

Both designs have their merits. The unified NOTES table offers simplicity and eliminates the need for additional tables. However, the alternative design provides clearer separation and potential for future extensibility.

A Supertype/Subtype Perspective

Consider adopting a supertype/subtype approach, where PUBLICATION serves as the supertype for BOOKS and ARTICLES. This allows for a single, shared NOTE table with a foreign key to PUBLICATION. This structure enables seamless retrieval of notes regardless of whether they are associated with a book or an article.

Benefits of the Supertype/Subtype Design:

• Flexibility: Adding a new entity, such as MAGAZINE, becomes straightforward by extending the PUBLICATION supertype.
• Data Integrity: The hierarchical relationship between PUBLICATION, BOOKS, ARTICLES, and NOTES ensures data consistency.

Example Structure:

TABLE PUBLICATION (
  ID (PK)

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