mysql - 为什么我不能以这种方式添加外键约束？

Question

表格:

    CREATE TABLE `relation` (
     `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
     `name` varchar(40) NOT NULL,
     `gender` tinyint(1) DEFAULT NULL,
     PRIMARY KEY (`id`),
     UNIQUE KEY `unique_relation` (`name`)
    ) ENGINE=InnoDB DEFAULT CHARSET=utf8

    CREATE TABLE `invite` (
     `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
     `date_sent` date NOT NULL,
     `user_id` int(10) unsigned NOT NULL,
     `relation_id` int(10) unsigned NOT NULL,
     `email` varchar(255) NOT NULL,
     `code` varchar(255) NOT NULL,
     PRIMARY KEY (`id`),
     KEY `fk_user` (`user_id`),
     CONSTRAINT `fk_user` FOREIGN KEY (`user_id`) REFERENCES `user` (`id`) ON DELETE CASCADE
    ) ENGINE=InnoDB DEFAULT CHARSET=utf8

执行的SQL语句是:

ALTER TABLE `invite` 
ADD CONSTRAINT `fk_relation`  
FOREIGN KEY (`relation_id`) 
REFERENCES `relation` (`id`) 
ON DELETE CASCADE ON UPDATE RESTRICT

Mysql 错误:

SQLSTATE[HY000]:一般错误:1005 无法创建表“dbtest.#sql-d00_39”(错误号:121)。

relation.id 和 invite.relation_id 列的类型相同 int(10) unsigned

更新 添加此键时，餐 table 邀请为空。 表关系有 3 行。

Answer 1

试试这个:

ALTER TABLE invite
ADD CONSTRAINT fk_relation
FOREIGN KEY (relation_id)
REFERENCES relation(id)

根据doc语法正确SQL FOREIGN KEY Constraint