Foreign Key Constraint Error in MySQL: Root Causes and Solutions
The error "ERROR 1215 (HY000): Cannot add foreign key constraint" arises when defining a foreign key reference between two tables in MySQL. To address this issue, it's crucial to ensure that specific criteria are met for both the referenced field and the foreign key field:
Engine Compatibility:
Both fields must reside in tables with the same engine, typically InnoDB.
Data Type and Length:
The data types and lengths of the fields must be identical. For instance, if the referenced field is a VARCHAR(20), the foreign key field should also be VARCHAR(20).
Collation:
The collation of the fields should match as well, for example, utf8.
Uniqueness:
The referenced field in the parent table should be unique or declared as PRIMARY KEY to ensure that each row in the child table has a valid reference.
NULL Constraints:
If the foreign key field allows NULL values, the referenced field should also allow NULL values.
Exclusive Execution:
Certain scenarios may require executing the foreign key creation statement exclusively without any other concurrent queries. This can be achieved by starting a transaction, for instance:
<code class="sql">BEGIN; CREATE TABLE course ( ... ); COMMIT;</code>
Additional Measures:
Remember, it's essential to verify that all these conditions are met to ensure successful foreign key constraint creation.
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