mysql> create table A(
    ->  Id int auto_increment primary key,
    ->  Name varchar(32) not null,
    ->  ParentId int,
    ->  FOREIGN KEY (ParentId)  references a(id)
    -> ) ENGINE=innodb;
Query OK, 0 rows affected (0.07 sec)

mysql> insert into a values (null,'AAAA',null);
Query OK, 1 row affected (0.07 sec)

mysql> insert into a values (null,'BBBB',3);
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint f
ails (`csdn`.`a`, CONSTRAINT `a_ibfk_1` FOREIGN KEY (`ParentId`) REFERENCES `a`
(`Id`))
mysql> insert into a values (null,'BBBB',1);
Query OK, 1 row affected (0.11 sec)

mysql>