mysql查询外键返回结果为空

There could be several reasons why a MySQL query returning a foreign key constraint could return an empty result:

1. The foreign key value being queried does not exist in the related table.
2. The foreign key value being queried is incorrectly specified in the query.
3. There are no rows in the related table that match the foreign key value being queried.
4. 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.