There could be several reasons why a MySQL query returning a foreign key constraint could return an empty result:
- The foreign key value being queried does not exist in the related table.
- The foreign key value being queried is incorrectly specified in the query.
- There are no rows in the related table that match the foreign key value being queried.
- There is a mistake in the query syntax that is preventing the correct results from being returned.
To troubleshoot and resolve this issue, you can check the data in the related tables, double-check the query syntax, and verify that the foreign key relationships are properly defined in the database schema.