Question

I have a table which has a “foreign key” referencing itself. This would be very useful, except I am uncertain how to add the first record to such a table. No matter what I add, I cannot provide a valid “foreign” key to the table itself, having no entries yet. Maybe I’m not going about this correctly, but I want this table to represent something that is always a member of itself. Is there a way to “bootstrap” such a table, or another way to go about self-reference?

Answer 1

One option is to make your field NULL-able, and set the root record’s parent key to NULL:

CREATE TABLE tb_1 (
   id       int   NOT NULL  PRIMARY KEY,
   value    int   NOT NULL,
   parent   int   NULL,
   FOREIGN KEY (parent) REFERENCES tb_1(id)
) ENGINE=INNODB;
Query OK, 0 rows affected (0.43 sec)

-- This fails:
INSERT INTO tb_1 VALUES (1, 1, 0);
ERROR 1452 (23000): A foreign key constraint fails.

-- This succeeds:
INSERT INTO tb_1 VALUES (1, 1, NULL);
Query OK, 1 row affected (0.08 sec)

Otherwise you could still use a NOT NULL parent key and point it to the root record itself:

CREATE TABLE tb_2 (
   id       int   NOT NULL  PRIMARY KEY,
   value    int   NOT NULL,
   parent   int   NOT NULL,
   FOREIGN KEY (parent) REFERENCES tb_2(id)
) ENGINE=INNODB;
Query OK, 0 rows affected (0.43 sec)

-- This fails:
INSERT INTO tb_2 VALUES (1, 1, 0);
ERROR 1452 (23000): A foreign key constraint fails.

-- This succeeds:
INSERT INTO tb_2 VALUES (1, 1, 1);
Query OK, 1 row affected (0.08 sec)