How to Troubleshoot 'Cannot add foreign key constraint' Errors in MySQL?

MySQL Foreign Key Constraint Error Troubleshooting Guide

Introduction

When establishing foreign key constraints for tables in the database, errors may be encountered during creation or modification. One such error is "ERROR 1215 (HY000): Cannot add foreign key constraint".

Case: Foreign key constraint issues in Patient and Appointment tables

The following SQL query encountered an error while trying to add a foreign key constraint to the Patient and Appointment tables:

<code class="language-sql">CREATE TABLE IF NOT EXISTS `doctorsoffice`.`Patient` (
...
  CONSTRAINT `FK_MedicalHistory`
    FOREIGN KEY (`MedicalHistory`)
    REFERENCES `doctorsoffice`.`medicalhistory` (`MedicalHistoryID`)
    ON DELETE CASCADE
    ON UPDATE CASCADE,
  CONSTRAINT `FK_PrimaryPhysician`
    FOREIGN KEY (`PrimaryPhysician`)
    REFERENCES `doctorsoffice`.`doctor` (`DoctorID`)
    ON DELETE CASCADE
    ON UPDATE CASCADE
);
CREATE TABLE IF NOT EXISTS `doctorsoffice`.`Appointment` (
...
  CONSTRAINT `FK_Patient`
    FOREIGN KEY (`Patient`)
    REFERENCES `doctorsoffice`.`Patient` (`PatientID`)
    ON DELETE CASCADE
    ON UPDATE CASCADE,
  CONSTRAINT `FK_Doctor`
    FOREIGN KEY (`Doctor`)
    REFERENCES `doctorsoffice`.`doctor` (`DoctorID`)
    ON DELETE CASCADE
    ON UPDATE CASCADE
);</code>

Solution: Verify data type compatibility

To resolve this issue, verify that the data types of the columns involved in the foreign key constraint are compatible. In this case, be sure to:

• The data type of the MedicalHistory column in the Patient table exactly matches the MedicalHistoryID column in the medicalhistory table, i.e. both should be of type INT.
• The data type of the PrimaryPhysician column in the Patient table matches the data type of the DoctorID column in the doctor table, both should be of type SMALLINT.

After correcting these data type mismatches, you can successfully add the foreign key constraint.

Additional Tip: Disable foreign key checking

To add more flexibility when creating tables in a specific order, consider using the following command before executing the SQL query:

<code class="language-sql">SET foreign_key_checks=0;</code>

This command temporarily disables foreign key checking, allowing tables to be created regardless of parent-child table dependencies. After the table is created, foreign key checking can be re-enabled using the following command:

<code class="language-sql">SET foreign_key_checks=1;</code>

By following these guidelines, you can ensure that foreign key constraints are implemented correctly in your database, preventing errors and maintaining data integrity.

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