mysql - mysql 中的外键约束。我无法修复它吗？

Question

我在mysql中创建了如下表:

CREATE TABLE employee (
  fname    varchar(15) not null, 
  minit    varchar(1),
  lname    varchar(15) not null,
  ssn      char(9),
  bdate    date,
  address  varchar(50),
  sex      char,
  salary   decimal(10,2),
  superssn char(9),
  dno      integer(4),
  primary key (ssn),
  foreign key (superssn) references employee(ssn)
);

然后我尝试插入以下信息:

INSERT INTO employee VALUES ("Bob","B","Bender","666666600",
"1968-04-17","8794 Garfield, Chicago, IL","M","96000.00","null","8");

不幸的是，我收到了这个错误:

Error Code: 1452. Cannot add or update a child row: a foreign key constraint 
fails (`test`.`employee`, CONSTRAINT `employee_ibfk_1` FOREIGN KEY 
(`superssn`) REFERENCES `employee` (`ssn`))

我尝试更改表，使 superssn 可以为空，但没有解决方案有效。

ALTER TABLE employee MODIFY COLUMN superssn CHAR(9) default null;

Answer 1

"null" 与 null 不同。删除引号。