CREATE TABLE a(a_key INT primary key, not_key INT);

CREATE TABLE b(for_key INT REFERENCES a(not_key));

SHOW CREATE TABLE b;

+-------+--------------------------------------------------------------------------------------------+

| Table | Create Table                                                                               |

+-------+--------------------------------------------------------------------------------------------+

| b     | CREATE TABLE `b` (

  `for_key` int(11) DEFAULT NULL

) ENGINE=InnoDB DEFAULT CHARSET=latin1 |

+-------+--------------------------------------------------------------------------------------------+

INSERT INTO a VALUES (1,10);

INSERT INTO b VALUES (10);

INSERT INTO b VALUES (1);

CREATE TABLE a(a_key INT primary key, not_key INT);

CREATE TABLE b(for_key INT REFERENCES a(not_key));

ERROR 1005 (HY000): Can't create table `test`.`b` (errno: 150 "Foreign key constraint is incorrectly formed")

Warning (Code 150): Create  table `test`.`b` with foreign key (for_key) constraint failed. There is no index in the referenced table where the referenced columns appear as the first columns.

Error (Code 1005): Can't create table `test`.`b` (errno: 150 "Foreign key constraint is incorrectly formed")

Warning (Code 1215): Cannot add foreign key constraint for `b`

CREATE TABLE c(for_key INT REFERENCES a(a_key));

INSERT INTO c VALUES (22);

ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`test`.`c`, CONSTRAINT `c_ibfk_1` FOREIGN KEY (`for_key`) REFERENCES `a` (`a_key`))

INSERT INTO c VALUES (1);

INSERT INTO c VALUES (2);

ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`test`.`c`, CONSTRAINT `c_ibfk_1` FOREIGN KEY (`for_key`) REFERENCES `a` (`a_key`))

SHOW CREATE TABLE c;

+-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

| Table | Create Table                                                                                                                                                                                      |

+-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

| c     | CREATE TABLE `c` (

  `for_key` int(11) DEFAULT NULL,

  KEY `for_key` (`for_key`),

  CONSTRAINT `c_ibfk_1` FOREIGN KEY (`for_key`) REFERENCES `a` (`a_key`)

) ENGINE=InnoDB DEFAULT CHARSET=latin1 |

+-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

INSERT INTO a VALUES (1,10);

INSERT INTO c VALUES (10);

ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`test`.`c`, CONSTRAINT `c_ibfk_1` FOREIGN KEY (`for_key`) REFERENCES `a` (`a_key`))

INSERT INTO c VALUES (1);

Query OK, 1 row affected (0.002 sec)