--source include/have_innodb.inc

CREATE TABLE parent ( id INT NOT NULL AUTO_INCREMENT, PRIMARY KEY (id)) ENGINE = InnoDB;
insert into parent values(1);
CREATE TABLE child ( id INT NOT NULL AUTO_INCREMENT, parent_fk INT DEFAULT NULL, PRIMARY KEY (id), CONSTRAINT fk_child_parent FOREIGN KEY (parent_fk) REFERENCES parent (id)) ENGINE = InnoDB;
INSERT INTO child values (1, NULL);
--error 1452
INSERT INTO child values (2, 0);
INSERT INTO child values (3, 1);
select * from child;

ALTER TABLE child MODIFY COLUMN parent_fk INT NOT NULL;
select * from child;