我在MariaDB 11.0.2中有以下两个表：

CREATE TABLE `Languages` (
  `Name` char(49) DEFAULT NULL,
  `ISO_639_1` char(2) NOT NULL,
  `Language_ID` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `Main_Flag` varchar(20) DEFAULT NULL,
  PRIMARY KEY (`Language_ID`),
  UNIQUE KEY `Languages_UN` (`ISO_369_1`,`Main_Flag`)
) ENGINE=InnoDB AUTO_INCREMENT=136 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;

字符串
和/或

CREATE TABLE `Tests` (
  `Test_ID` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `Test_Name` varchar(50) DEFAULT NULL,
  `ISO_639_1` char(2) NOT NULL,
  `Main_Flag` varchar(20) DEFAULT NULL,
  PRIMARY KEY (`Test_ID`)
) ENGINE=InnoDB AUTO_INCREMENT=136 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;

型
但我不能将双列唯一键分配给它们的对应项：

MariaDB [my_db]> ALTER TABLE  Tests ADD CONSTRAINT Test_Language_FK FOREIGN KEY (ISO_639_1, Main_Flag) REFERENCES Languages(ISO_639_1, Main_Flag);
ERROR 1005 (HY000): Can't create table `my_db`.`Tests` (errno: 150 "Foreign key constraint is incorrectly formed")

型
测试表的索引没有异常：

MariaDB [my_db]> show index from Tests;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Ignored |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+
| Tests |          0 | PRIMARY  |            1 | Test_ID     | A         |          37 |     NULL | NULL   |      | BTREE      |         |               | NO      |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+
1 row in set (0,001 sec)

型
是什么在阻碍我？数据类型相同，排序规则和字符集也匹配。