When you execute queries to check for the constraint fk_capital, it may not return results because Oracle treats it as FK_CAPITAL internally.

Steps to Resolve the Issue:

1. Verify the Constraint Name: Run the following query to check for constraints in uppercase:

SELECT owner, constraint_type, table_name FROM user_constraints WHERE constraint_name = 'FK_CAPITAL';

2. Check All Constraints for the Table: If the above query doesn't work, list all constraints for the nations table to confirm its existence:

SELECT constraint_name, constraint_type FROM user_constraints WHERE table_name = UPPER('NATIONS');

3. Dropping the Constraint (If Necessary): If the fk_capital constraint exists, you can drop it using:

ALTER TABLE nations DROP CONSTRAINT fk_capital;

4. Recreate the Constraint: Once confirmed or dropped, you can recreate the constraint:

ALTER TABLE nations ADD CONSTRAINT fk_capital FOREIGN KEY (capital) REFERENCES cities(name);

Additional Tips:

Ensure No Duplicate Constraints: Confirm that no other constraint is using the name fk_capital across different schemas.

SELECT owner, table_name, constraint_name FROM all_constraints WHERE constraint_name = 'FK_CAPITAL';

Schema Ownership: If you are working in a multi-schema environment, ensure you are querying the correct schema by prefixing the table name with the schema name (e.g., schema_name.nations).

If the issue persists, it might be related to the Oracle free version limitations or a database bug.