CREATE TABLE t1 (a INT primary key, b int) ENGINE=InnoDB;

CREATE TABLE t2 (a int primary key, b int, constraint b foreign key(b) references t1(a)) engine=innodb;

SHOW CREATE TABLE t2;

Table	Create Table

t2	CREATE TABLE `t2` (

  `a` int(11) NOT NULL,

  `b` int(11) DEFAULT NULL,

  PRIMARY KEY (`a`),

  KEY `b` (`b`),

  CONSTRAINT `b` FOREIGN KEY (`b`) REFERENCES `t1` (`a`)

) ENGINE=InnoDB DEFAULT CHARSET=latin1

ALTER TABLE t2 DROP CONSTRAINT b, ALGORITHM=COPY;

SHOW CREATE TABLE t2;

Table	Create Table

t2	CREATE TABLE `t2` (

  `a` int(11) NOT NULL,

  `b` int(11) DEFAULT NULL,

  PRIMARY KEY (`a`),

  KEY `b` (`b`),

  CONSTRAINT `b` FOREIGN KEY (`b`) REFERENCES `t1` (`a`)

) ENGINE=InnoDB DEFAULT CHARSET=latin1