<转>mysql在同一张表外键

mysql> create table test4(a int primary key,b int,foreign key (b) references test4(a));
Query OK, 0 rows affected (0.12 sec)

mysql> show create table test4;
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                   |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| test4 | CREATE TABLE `test4` (
  `a` int(11) NOT NULL,
  `b` int(11) DEFAULT NULL,
  PRIMARY KEY (`a`),
  KEY `b` (`b`),
  CONSTRAINT `test4_ibfk_1` FOREIGN KEY (`b`) REFERENCES `test4` (`a`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.02 sec)

mysql> insert into test4 values(1,1);
Query OK, 1 row affected (0.02 sec)

mysql> insert into test4 values(1,2);
ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'