不能删除列：在外键约束中需要

Question

我有一个带有两个外键约束的表，如下所示：

mysql> SHOW CREATE TABLE `user`;

CREATE TABLE `user` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `region_id` int(11) unsigned DEFAULT NULL,
  `town_id` int(11) unsigned DEFAULT NULL,
  `fullname` varchar(50) COLLATE utf8_unicode_ci DEFAULT NULL,
  `username` varchar(30) COLLATE utf8_unicode_ci DEFAULT NULL,
  `email` varchar(100) COLLATE utf8_unicode_ci DEFAULT NULL,
  `password` varchar(50) COLLATE utf8_unicode_ci DEFAULT NULL,
  `active` tinyint(1) unsigned NOT NULL DEFAULT '0',
  PRIMARY KEY (`id`),
  KEY `FK_G38T6P7EKUXYWH1` (`region_id`),
  KEY `FK_J8VWK0ZN7FD2QX4` (`town_id`),
  CONSTRAINT `FK_G38T6P7EKUXYWH1` FOREIGN KEY (`region_id`) REFERENCES `region` (`id`) ON UPDATE NO ACTION,
  CONSTRAINT `FK_J8VWK0ZN7FD2QX4` FOREIGN KEY (`town_id`) REFERENCES `town` (`id`) ON UPDATE NO ACTION
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci

尽管我禁用了FOREIGN_KEY_CHECKS.

mysql> ALTER TABLE `user` DROP COLUMN `region_id`;
1553 - Cannot drop index 'FK_G38T6P7EKUXYWH1': needed in a foreign key constraint

mysql> SHOW VARIABLES LIKE 'FOREIGN_KEY%';
+--------------------+-------+
| Variable_name      | Value |
+--------------------+-------+
| foreign_key_checks | ON    |
+--------------------+-------+
1 row in set

mysql> SET FOREIGN_KEY_CHECKS = 0;
Query OK, 0 rows affected

mysql> SHOW VARIABLES LIKE 'FOREIGN_KEY%';
+--------------------+-------+
| Variable_name      | Value |
+--------------------+-------+
| foreign_key_checks | OFF   |
+--------------------+-------+
1 row in set

mysql> ALTER TABLE `user` DROP COLUMN `region_id`;
1828 - Cannot drop column 'region_id': needed in a foreign key constraint 'FK_G38T6P7EKUXYWH1'

Answer 1

看看MySql 文档，我发现了一个警告foreign_key_keys：

警告 如果
foreign_key_checks=0，删除外键约束所需的索引会使表处于不一致状态并导致表加载时发生的外键检查失败。为避免此问题，请在删除索引之前删除外键约束（错误 #70260）。

恕我直言，您应该在删除列之前删除外键。

ALTER TABLE `user` DROP FOREIGN KEY `FK_G38T6P7EKUXYWH1`;
ALTER TABLE `user` DROP COLUMN `region_id`;

我已经设置了一个 rextester 示例，请在此处查看。