Why Am I Getting a Foreign Key Constraint Error 1215?

Foreign Key Constraint Error: 1215

When attempting to create database tables with foreign key constraints, an error code 1215 can occur. This indicates an issue with adding a foreign key constraint, often due to a datatype mismatch.

Detailed Analysis:

The error message suggests that the foreign key column classLeader in the class table cannot reference the primary key studentID in the student table because their datatypes don't match. In the provided database setup, classLeader is a VARCHAR (string) type, while studentID is an INT (integer) type. Foreign key references require a matching datatype for data integrity validation.

Alternatives:

To resolve the error, the datatype of classLeader should be changed to INT or vice versa, depending on the business requirements. If storing actual student names in classLeader is crucial, an additional join table or a unique index on the firstName and lastName columns of the student table can be used to ensure data consistency.

Filling Tables with Foreign Keys:

Tables with foreign key constraints have a specific filling order. The primary table (student) should be filled before the referencing table (class), or foreign key constraint violations will occur. When filling the class table, the classLeader value must reference a valid studentID in the student table.

Nature of Foreign Keys:

Foreign keys are not primary or unique keys themselves. They are used to establish a relationship between two tables, ensuring that data integrity remains intact. In this case, the classLeader foreign key ensures that every class has a valid studentID assigned to it, maintaining the data consistency between the two tables.

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