How do I create a table with self-referencing fields in MySQL?

Question

In GTFS, the field parent_station is either from stop_id (self-reference) or NULL. For instance (note that parent_station might be NULL),

SELECT stop_id, stop_name, parent_station FROM stops where stop_id='1970324837184617' OR parent_station='1970324837184617';

+------------------+----------------------+------------------+
| stop_id          | stop_name            | parent_station   |
+------------------+----------------------+------------------+
| 1970324837184617 | Saint Agne-SNCF      |                  |
| 3377699720880648 | Saint Agne-SNCF      | 1970324837184617 |
| 3377699720880649 | Saint Agne-SNCF      | 1970324837184617 |
| 3377699722011100 | Saint Agne-SNCF      | 1970324837184617 |
| 3377699722011101 | Saint Agne-SNCF      | 1970324837184617 |
| 3377699722914835 | Saint Agne Gare SNCF | 1970324837184617 |
+------------------+----------------------+------------------+
6 rows in set (0,01 sec)

I create a self-referencing table with:

CREATE TABLE `stops` (
    stop_id VARCHAR(255) NOT NULL PRIMARY KEY,
    stop_code VARCHAR(255),
    stop_name VARCHAR(255),
    stop_desc VARCHAR(255),
    stop_lat DECIMAL(8,6),
    stop_lon DECIMAL(8,6),
    location_type INT(2),
    parent_station VARCHAR(255),
    -- the field `parent_station` might be blank (NULL)
    FOREIGN KEY parent_station REFERENCES stops(stop_id)  -- self-referential table fields
);

but encounter the issue,

ERROR 1064 (42000) at line 99: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'FOREIGN KEY parent_station REFERENCES stops(stop_id)
)' at line 11

---

With the help of @Awita, I correct the syntax and the table is created successfully.

FOREIGN KEY (parent_station) REFERENCES stops(stop_id) 

But a new issue occurs while loading local data.

LOAD DATA LOCAL INFILE 'stops.txt' INTO TABLE stops FIELDS TERMINATED BY ',' IGNORE 1 LINES;

The error:

ERROR 1452 (23000) at line 140: Cannot add or update a child row: a foreign key constraint fails (`paris_gtfs`.`stops`, CONSTRAINT `stops_ibfk_1` FOREIGN KEY (`parent_station`) REFERENCES `stops` (`stop_id`))

This issue is caused by NULL parent_station?

Answer 1

It's a simple syntax error. Try this:

FOREIGN KEY (parent_station) REFERENCES stops(stop_id) 

Answer 2

I tried to add the foreign key after the table has been created with this command and it works:

ALTER TABLE stops
ADD FOREIGN KEY (parent_station)
REFERENCES stops(stop_id)