MySQL 1215: Cannot add foreign key constraint.

When encountering error 1215: Cannot add foreign key constraint in MySQL, follow these steps:

  1. Check for Typo in Table or Column Names:


- Ensure the table and column names in the foreign key constraint are spelled correctly.

  1. Verify Table Engine:


- Both tables need to use the InnoDB storage engine. Check with:
SHOW TABLE STATUS WHERE Name = 'yourtablename';

  1. Ensure Matching Column Types:


- The columns you are trying to reference must have the exact same data type and length. Check column types with:
DESCRIBE yourtablename;

  1. Check for Existing Data Violations:


- Ensure that the existing data does not violate the foreign key constraint you are trying to create.

  1. Confirm Foreign Key Columns are Indexed:


- The referenced column(s) in the parent table must be indexed. Check indexes with:
SHOW INDEX FROM yourtablename;

  1. Check for Duplicate Foreign Key Names:


- Ensure that the foreign key name is unique across the database.

  1. Review MySQL Error Log:


- Look into the MySQL error log for any additional messages related to this error.

  1. Try Creating the Foreign Key with a Simplified Syntax:


- Temporarily remove any `ON DELETE` or `ON UPDATE` clauses to isolate the issue.

  1. Check Foreign Key Constraints for the Referenced Table:


- Ensure there are no conflicting constraints on the referenced table that would prevent the addition of the new foreign key.

  1. Execute SHOW ENGINE INNODB STATUS:


- For a detailed explanation of the latest InnoDB foreign key error in the server, run:
SHOW ENGINE INNODB STATUS;
Look under the `LATEST FOREIGN KEY ERROR` section.

Take action based on findings from these steps to resolve the specific cause of the error 1215 in MySQL.

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