Can MySQL Enforce Conditional Foreign Key Constraints for Polymorphic Associations?

MySQL conditional foreign key constraints: challenges and solutions

In database design, data integrity is crucial. Foreign key constraints are used to maintain relationships between tables, but in some cases it may be necessary to conditionally enforce constraints based on specific column values. For example, design a "Comments" table to store comments for various entities such as blog posts and user pictures.

Does MySQL support conditional foreign key constraints?

Not supported. MySQL's foreign key constraints must explicitly reference a single table, ensuring that every row in the referencing table has a matching row in the referenced table. Conditional constraints violate the principles of relational database design.

Solution: Polymorphic Association and "Super Table"

It is recommended to use "polymorphic association" technology to solve this problem. This technique involves creating a "hypertable" that stores the common attributes of all entities referenced by foreign keys. In this example, create a "Commentable Entities" table:

<code class="language-sql">CREATE TABLE Commentable (
  id SERIAL PRIMARY KEY
);</code>

Your entities (such as "Blog Post" and "User Image") will then be subtypes of this hypertable:

<code class="language-sql">CREATE TABLE BlogPosts (
  blogpost_id INT PRIMARY KEY, -- 注意，此 ID 不是自动生成的
  ...
  FOREIGN KEY (blogpost_id) REFERENCES Commentable(id)
);

CREATE TABLE UserPictures (
  userpicture_id INT PRIMARY KEY, -- 注意，此 ID 不是自动生成的
  ...
  FOREIGN KEY (userpicture_id) REFERENCES Commentable(id)
);</code>

Implementation details:

• Insert a new row into the "Commentable Entities" table to generate a unique "id" before inserting the new entity into the subtype table.
• Use the generated "id" as a "foreign key" when inserting into the subtype table.

This approach ensures referential integrity while maintaining the flexibility to accommodate a variety of entity types and store comments in a single "Comments" table.

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