Single Column Foreign Key Reference Multiple Columns?

Introduction

In this article, we’ll explore the concept of a single column foreign key referencing multiple columns in a database. This can be a challenging problem to solve, especially when dealing with existing table structures that cannot be easily modified.

We’ll examine a specific Stack Overflow question and provide a detailed explanation of how to achieve this goal using SQL Server and MySQL.

The Problem

The original question presents two tables: pieces and workings. The pieces table contains serial numbers represented by definitiveCode and internalCode, while the workings table has an additional column pieceCode that represents the serial number of the item on which the working was performed.

The goal is to create a foreign key constraint that ensures the pieceCode in the workings table is present in either the definitiveCode or internalCode columns in the pieces table. This would make it easier to insert and remove works from the system, as well as ensure data consistency.

Designing the Solution

To solve this problem, we’ll need to create a new table that acts as a bridge between the two existing tables. This bridge table will contain foreign keys referencing both pieces and workings.

Step 1: Create the Bridge Table

The first step is to create a new table called piececodes. This table will serve as the bridge between the two existing tables.

CREATE TABLE piececodes (
    piececode INT,
    PRIMARY KEY (piececode)
);

This table has a single column, piececode, which will be used as the foreign key referencing both pieces and workings.

Step 2: Create Foreign Keys in Pieces Table

Next, we’ll modify the pieces table to create foreign keys referencing the new piececodes table.

CREATE TABLE pieces (
    definitivecode INT,
    internalcode INT,
    CONSTRAINT pieces_unique_definitivecode UNIQUE (definitivecode),
    CONSTRAINT pieces_unique_internalcode UNIQUE (internalcode),
    FOREIGN KEY (definitivecode) REFERENCES piececodes(piececode),
    FOREIGN KEY (internalcode) REFERENCES piececodes(piececode)
);

By creating foreign keys in the pieces table, we ensure that any changes made to the piececodes table will be reflected in both tables.

Step 3: Create Foreign Key in Workings Table

We’ll also create a foreign key in the workings table referencing the new piececodes table.

CREATE TABLE workings (
    piececode INT,
    ...
    FOREIGN KEY (piececode) REFERENCES piececodes(piececode)
);

This ensures that any changes made to the pieces or piececodes tables will be reflected in the workings table.

Handling Composite Keys

If we want to ensure that a piece code can only be used as either a definite code or an internal code, things get more complicated. In this case, we’ll need to create a new table called piececodetypes and modify our existing tables accordingly.

CREATE TABLE piececodetypes (
    piececodetype VARCHAR(10),
    CONSTRAINT piececodetypes_check_type CHECK (piececodetype IN ('DEFINITE', 'INTERNAL')),
    PRIMARY KEY (piececodetype)
);

CREATE TABLE piececodes (
    piececode INT,
    piececodetype VARCHAR(10),
    PRIMARY KEY (piececode, piececodetype),
    FOREIGN KEY (piececodetype) REFERENCES piececodetypes(piececodetype)
);

We’ll also modify our existing tables to include foreign keys referencing the new piececodetypes table.

CREATE TABLE pieces (
    definitivecode INT,
    internalcode INT,
    definitivecodetype  AS CAST('DEFINITE' AS VARCHAR(10)) PERSISTED,
    internalcodetype AS CAST('INTERNAL' AS VARCHAR(10)) PERSISTED,
    CONSTRAINT pieces_unique_definitivecode UNIQUE (definitivecode),
    CONSTRAINT pieces_unique_internalcode UNIQUE (internalcode),
    FOREIGN KEY (definitivecode, definitivecodetype) REFERENCES piececodes(piececode, piececodetype),
    FOREIGN KEY (internalcode, internalcodetype) REFERENCES piececodes(piececode, piececodetype)
);

CREATE TABLE workings (
    piececode INT,
    ...
    FOREIGN KEY (piececode) REFERENCES piececodes(piececode, piececodetype)
);

This design ensures that any changes made to the pieces or piececodes tables will be reflected in both tables.

Conclusion

In this article, we’ve explored the concept of a single column foreign key referencing multiple columns in a database. We’ve examined a specific Stack Overflow question and provided a detailed explanation of how to achieve this goal using SQL Server and MySQL.

By creating a new bridge table and modifying existing tables accordingly, we can create a robust data model that ensures consistency and accuracy. The solution presented handles composite keys with ease, providing a scalable and maintainable design for any database system.

Last modified on 2024-06-07