How to Avoid MySQL Errno 150 Errors When Using Foreign Key Constraints?

MySQL Foreign Key Constraints: Avoiding Errno 150 Errors

While creating a table in MySQL with foreign keys referencing other tables, you may encounter an errno: 150 error. To resolve this issue, it's crucial to adhere to the following guidelines:

1. Ensure Table Order: Define the parent table containing the primary key before creating the child table with the foreign key reference.
2. Enable Foreign Key Support: Both the parent and child tables should support foreign key constraints, typically by using the InnoDB storage engine.
3. Left-Most Column Key: The referenced columns in the parent table must be the left-most columns of a key, ideally a PRIMARY KEY or UNIQUE KEY.
4. Matching Primary Key Order: The FK definition must align with the PK definition in terms of column order and data types.
5. Proper Data Alignment: The PK columns in the parent table and the FK columns in the child table must match in terms of data type and collation.
6. Existing Data Compatibility: If the child table contains data, ensure that each FK column value matches a corresponding value in the parent table PK columns.
7. Avoid Temporary or Partitioned Tables: Foreign key constraints cannot be defined on TEMPORARY or PARTITIONED tables.
8. Nullable Columns: If an ON DELETE SET NULL option is used, the FK columns must be nullable.
9. Unique Constraint Names: Foreign key constraints should have unique names across the schema to avoid conflicts.
10. Dependent Foreign Keys: If a malformed FK exists in another table referencing the same field in the parent table, ensure consistency before creating the new FK. Use the following query to identify problematic FK's:

SELECT * FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE
WHERE CONSTRAINT_SCHEMA = 'your_schema_name'
AND CONSTRAINT_NAME  'PRIMARY' AND REFERENCED_COLUMN_NAME IS NOT NULL;

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