MySQL 外键阻止删除表 - 但表是空的

Question

为什么删除空表会产生外键错误？

在 MySQL 中，我有一个具有以下定义的表

CREATE TABLE `enterprise_rma` (
  `entity_id` int(10) unsigned NOT NULL AUTO_INCREMENT COMMENT 'RMA Id',
  `status` varchar(32) DEFAULT NULL COMMENT 'Status',
  `is_active` smallint(5) unsigned NOT NULL DEFAULT '1' COMMENT 'Is Active',
  `increment_id` varchar(50) DEFAULT NULL COMMENT 'Increment Id',
  `date_requested` timestamp NULL DEFAULT CURRENT_TIMESTAMP COMMENT 'RMA Requested At',
  `order_id` int(10) unsigned NOT NULL COMMENT 'Order Id',
  `order_increment_id` varchar(50) DEFAULT NULL COMMENT 'Order Increment Id',
  `store_id` smallint(5) unsigned DEFAULT NULL COMMENT 'Store Id',
  `customer_id` int(10) unsigned DEFAULT NULL,
  `customer_custom_email` varchar(255) DEFAULT NULL COMMENT 'Customer Custom Email',
  PRIMARY KEY (`entity_id`),
  KEY `IDX_ENTERPRISE_RMA_STATUS` (`status`),
  KEY `IDX_ENTERPRISE_RMA_IS_ACTIVE` (`is_active`),
  KEY `IDX_ENTERPRISE_RMA_INCREMENT_ID` (`increment_id`),
  KEY `IDX_ENTERPRISE_RMA_DATE_REQUESTED` (`date_requested`),
  KEY `IDX_ENTERPRISE_RMA_ORDER_ID` (`order_id`),
  KEY `IDX_ENTERPRISE_RMA_ORDER_INCREMENT_ID` (`order_increment_id`),
  KEY `IDX_ENTERPRISE_RMA_STORE_ID` (`store_id`),
  KEY `IDX_ENTERPRISE_RMA_CUSTOMER_ID` (`customer_id`),
  CONSTRAINT `FK_ENTERPRISE_RMA_CUSTOMER_ID_CUSTOMER_ENTITY_ENTITY_ID` FOREIGN KEY (`customer_id`) REFERENCES `customer_entity` (`entity_id`) ON DELETE SET NULL ON UPDATE CASCADE,
  CONSTRAINT `FK_ENTERPRISE_RMA_STORE_ID_CORE_STORE_STORE_ID` FOREIGN KEY (`store_id`) REFERENCES `core_store` (`store_id`) ON DELETE SET NULL ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='RMA LIst';

我正在尝试使用DROP此表，但每当我这样做时，都会收到以下错误。

无法删除或更新父行：外键约束失败

我不明白的是，桌子已经空了。如果 I SELECT * FROM enterprise_rma;，则不返回任何行 ( 0 rows affected)

并SHOW ENGINE INNODB STATUS;没有返回任何有用的东西。

I know I can work around this with the following

SET FOREIGN_KEY_CHECKS=0; 
DROP TABLE enterprise_rma;
SET FOREIGN_KEY_CHECKS=0; 

But I don't understand how a foreign key constraint could fail if there's no data in the table. Is there a subtlety of MySQL's foreign key mode I don't understand? Can MySQL get damaged or put in an invalid state when this happens? Are there methods for troubleshooting this myself?

Answer 1

You shouldn't be able to drop a table that is the parent of another dependent table, even if it has no data. Because if you drop the parent, then you will never be able to insert any data in the child. You must first drop any child tables that reference enterprise_rma.

Something doesn't match your description, though. You said you were having trouble dropping the table, but your example about the foreign key checks shows you using DELETE.

DELETE不一样DROP TABLE。之后DELETE，表中没有行，但表仍然存在。之后DROP TABLE，该表不存在，即如果您运行 ，它不会显示SHOW TABLES。