更改表时出错，添加约束外键获取错误“无法添加或更新子行"

2022-01-20 00:00:00 foreign-keys mysql 
mysql> DESCRIBE questions;
+----------+--------------+------+-----+---------+----------------+
| Field    | Type         | Null | Key | Default | Extra          |
+----------+--------------+------+-----+---------+----------------+
| id       | int(255)     | NO   | PRI | NULL    | auto_increment |
| question | varchar(255) | NO   |     | NULL    |                |
| type     | char(1)      | YES  |     | NULL    |                |
+----------+--------------+------+-----+---------+----------------+
mysql> DESCRIBE answers;  
+--------------+--------------+------+-----+---------+----------------+
| Field        | Type         | Null | Key | Default | Extra          |
+--------------+--------------+------+-----+---------+----------------+
| id           | int(255)     | NO   | PRI | NULL    | auto_increment |
| answer       | varchar(255) | NO   |     | NULL    |                |
| questionid   | int(255)     | NO   |     | NULL    |                |
| questions_id | int(255)     | NO   |     | NULL    |                |
+--------------+--------------+------+-----+---------+----------------+

我正在使用这种说法:

ALTER TABLE 答案添加外键(questions_id)参考问题(id)；

ALTER TABLE answers ADD FOREIGN KEY(questions_id) REFERENCES questions(id); 

但我得到这个错误:

ERROR 1452 (23000):无法添加或更新子行:外键约束失败(surveydb.#sql-df_32，CONSTRAINT #sql-df_32_ibfk_1 FOREIGN KEY (questions_id) REFERENCES questions (id)) 到您的 MySQL 服务器版本，以便在附近使用正确的语法第 1 行的描述问题"

ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (surveydb.#sql-df_32, CONSTRAINT #sql-df_32_ibfk_1 FOREIGN KEY (questions_id) REFERENCES questions (id))to your MySQL server version for the right syntax to use near 'DESCREBE questions' at line 1

推荐答案

answers.questions_id 中至少有一个数据值在 questions.id 中没有出现.

You have at least one data value in answers.questions_id that does not occur in questions.id.

这是我的意思的一个例子:

Here's an example of what I mean:

mysql> create table a ( id int primary key);

mysql> create table b ( aid int );

mysql> insert into a values (123);

mysql> insert into b values (123), (456);

mysql> alter table b add foreign key (aid) references a(id);
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint 
fails (`test`.`#sql-3dab_e5c`, CONSTRAINT `#sql-3dab_e5c_ibfk_1` FOREIGN KEY
(`aid`) REFERENCES `a` (`id`))

您可以使用它来确认存在不匹配的值:

You can use this to confirm that there are unmatched values:

SELECT COUNT(*)
FROM answers AS a
LEFT OUTER JOIN questions AS q ON a.questions_id = q.id
WHERE q.id IS NULL

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