What are the reasons why MySQL cannot add foreign keys?

There could be several reasons why MySQL is unable to add foreign keys.

1. The mismatch of data types: the data type of the column referenced by the foreign key does not match the data type of the referenced column, such as differing lengths or types.
2. Index issue: the column referenced by a foreign key does not have an index established, or the index does not meet certain conditions.
3. Mismatched values: the column referenced by the foreign key contains values that do not meet the requirements of the reference column, such as null values, duplicate values, etc.
4. Constraint conflict: There is already data existing on the referenced column or the reference column that violates the foreign key constraint.
5. The table engine does not support foreign keys: In the case of using the MyISAM table engine, MySQL does not support foreign key constraints.
6. Data integrity issue: There are data integrity problems on the columns referenced by foreign keys, such as the presence of null values on the column referenced by the foreign key.
7. There is an issue with the table status: it is either locked or being affected by other concurrent operations, making it impossible to add a foreign key constraint.