Resolving the Multiple Foreign Key Issue in MySQL: A Step-by-Step Guide

When working with databases, specifically in MySQL, you might encounter scenarios where you need to implement multiple foreign keys referencing the same parent table. This can be particularly challenging, as seen in the problem many users face, such as trying to associate different suppliers within a shipping details table. In this post, we will explore a common question regarding multiple foreign keys, provide a structured solution encompassing best practices, and guide you through the troubleshooting process.

The Problem

Imagine you have a SHIPPING_DETAILS table that is responsible for tracking both the shipping vendors (like FedEx or UPS) and the product handling vendors (think Dunder Mifflin). In this situation, multiple columns within the shipping details need to reference the same parent table, VENDOR. However, issues can arise when you try to create foreign key constraints for these columns, such as being unable to define multiple foreign keys.

Users often experience errors like the following:

Can't create table './<DB_NAME>/SHIPPING_GRID.frm' (errno: 150)

This error typically signals a problem with the foreign key definition or the data types involved.

Solution Overview

To solve the problem of defining multiple foreign keys in MySQL, follow these steps:

1. Define Table Structure Properly: Ensure that your VENDOR table has a unique primary key that is compatible with the foreign key columns in your SHIPPING_DETAILS table.

2. Correct the Table Type: Make sure both tables (VENDOR and SHIPPING_GRID) are of type InnoDB. This is crucial as foreign key constraints are not supported in other storage engines like MyISAM.

3. Ensure Data Type Compatibility: Verify that the data types of the foreign keys match the data type of the primary key they reference. For instance, if VENDOR.no is defined as INT(6), all foreign key references must also be of the same type.

4. Avoid Duplicate Primary Key Definitions: Ensure that your SQL statement does not inadvertently include multiple definitions of the primary key. This can lead to errors during table creation.

Example Code

Here is a corrected example of how your SHIPPING_GRID table can be defined correctly after considering the points above:

CREATE TABLE SHIPPING_GRID(  
    id INT NOT NULL AUTO_INCREMENT PRIMARY KEY COMMENT 'Unique ID for each row',
    shipping_vendor_no INT(6) NOT NULL COMMENT 'Foreign key to VENDOR.no for the shipping vendor (vendors_type must be 3)',
    start_vendor_no INT(6) NOT NULL COMMENT 'Foreign key to VENDOR.no for the vendor being shipped from',
    end_vendor_no INT(6) NOT NULL COMMENT 'Foreign key to the VENDOR.no for the vendor being shipped to',
    shipment_duration INT(1) DEFAULT 1 COMMENT 'Duration in whole days shipment will take',
    price FLOAT(5,5) NOT NULL COMMENT 'Price in US dollars per shipment lbs (down to 5 decimal places)',
    is_flat_rate TINYINT(1) DEFAULT 0 COMMENT '1 if is flat rate regardless of weight, 0 if price is by lbs',
    INDEX (shipping_vendor_no),
    INDEX (start_vendor_no),
    INDEX (end_vendor_no),
    FOREIGN KEY (shipping_vendor_no) REFERENCES VENDOR (no),
    FOREIGN KEY (start_vendor_no) REFERENCES VENDOR (no),
    FOREIGN KEY (end_vendor_no) REFERENCES VENDOR (no)  
) ENGINE=InnoDB;

Key Takeaways

• Data Types Matter: Always ensure that the foreign key columns match the data type of the referenced primary key.
• Use InnoDB: If you are setting up foreign keys across tables, using the InnoDB engine is essential.
• Avoid Duplicates: If you find your SQL is generating errors, double-check for duplicate definitions or syntax issues.

By following these guidelines, you can effectively create your table with multiple foreign keys and avoid common pitfalls associated with foreign key constraints. Good luck with your MySQL endeavors and happy coding!