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Mysql外键约束_MySQL

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May 27, 2016 pm 01:45 PM

Mysql集群创建外键，分为四种约束：no action，restrict，cascade，set null。如果表A的主关键字是表B中的字段，则该字段称为B的外键，表A称为主表，表B称为从表。外键是用来实现参照完整性的，不同的外键不同的外键约束方式将可以使两张表紧密的结合起来，特别是修改或者删除的级联操作将使得日常的维护工作更加轻松。

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CASCADE: 从父表删除或更新且自动删除或更新子表中匹配的行。

·SET NULL: 从父表删除或更新行，并设置子表中的外键列为NULL。

·NO ACTION: 在ANSI SQL-92标准中，NO ACTION意味这不采取动作，就是如果有一个相关的外键值在被参考的表里，删除或更新主要键值的企图不被允许进行（Gruber, 掌握SQL, 2000:181）。 InnoDB拒绝对父表的删除或更新操作。

RESRICT:同no action。

环境模拟：

先来添加两张表：ta_resource和ta_resourcetime。

DROP TABLE IF EXISTS `ta_resource`;
CREATE TABLE `ta_resource` (
  `ResourceId` varchar(64) NOT NULL,
  `ResourcePId` varchar(64) DEFAULT NULL,
  `ResourceName` varchar(64) DEFAULT NULL,
  `Type` varchar(64) DEFAULT NULL,
  `Desc` varchar(512) DEFAULT NULL,
  `Priority` smallint(6) DEFAULT NULL,
  `IsFold` varchar(2) DEFAULT NULL,
  `IsUse` varchar(2) DEFAULT NULL,
  `URL` varchar(64) DEFAULT NULL,
  `Icon` varchar(512) DEFAULT NULL,
  `Css` varchar(64) DEFAULT NULL,
  `JavaScript` varchar(64) DEFAULT NULL,
  `TimeSpan` datetime DEFAULT NULL,
  `IsDefault` varchar(2) DEFAULT NULL,
  PRIMARY KEY (`ResourceId`)
) ENGINE=ndbcluster DEFAULT CHARSET=utf8;

-- ----------------------------
--  Table structure for `ta_resourcetime`
-- ----------------------------
DROP TABLE IF EXISTS `ta_resourcetime`;
CREATE TABLE `ta_resourcetime` (
  `TimeId` varchar(64) NOT NULL,
  `ResourceId` varchar(64) NOT NULL,
  `ResourceName` varchar(64) DEFAULT NULL,
  `RoleId` varchar(64) NOT NULL,
  `RoleName` varchar(64) DEFAULT NULL,
  `URL` varchar(64) DEFAULT NULL,
  `StartTime` datetime NOT NULL,
  `EndTime` datetime NOT NULL,
  `IsDeleted` varchar(2) DEFAULT NULL,
  `Desc` varchar(512) DEFAULT NULL,
  PRIMARY KEY (`TimeId`),
  KEY `ResourceId` (`ResourceId`),
  CONSTRAINT `resourceid` FOREIGN KEY (`ResourceId`) REFERENCES `ta_resource` (`ResourceId`) ON 
  DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=ndbcluster DEFAULT CHARSET=utf8;

1、no action时

这两个表中的外键使用的是ON DELETE NO ACTION ON UPDATE NO ACTION

如果更改数据时， ta_resource是主表，ta_resourcetime是从表。

更新数据时：可以修改主表，但是不可以修改从表的数据。

删除数据时：先删除从表，再删除主表。

2、cascade时

集群搭建完再插入数据库表时，在on delete 和on update 都使用cascade时遇到了这样的问题：不能添加外键约束。

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之前以为是mysql集群不支持外键，在网上查了mysql集群该版本支持外键约束和师哥们讨论过这个问题，表示mysql集群支持外键约束。于是经过多次实践，找到出现这种问题的原因：mysql集群中支持使用cascade，但是on delete 和on update只能有一个使用cascade，另一个使用no action就可以。

3、内存不足

出现的问题：插入不进去数据，经过查询，是由于内存不足

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解决方法：

在配置文件config.ini中对内存进行设置，主要对这几项进行设置：

MaxNoOfTables=10240

设置集群中最大表对象数量
MaxNoOfAttributes=500000

设置可在集群中定义的属性数量

MaxNoOfConcurrentTransactions=1000000
用于设定节点内可能的并发事务数
MaxNoOfConcurrentOperations=1000000
设置能同时出现在更新阶段或同时锁定的记录数
MaxNoOfOrderedIndexes=10240
设置有序索引的总数

如果不配置这些，mysql集群中会默认内存的大小，当内存满了以后就插不进去数据了。

以上就是Mysql外键约束_MySQL的内容，更多相关内容请关注PHP中文网（m.sbmmt.com）！

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