Mysql外键约束_MySQL
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。
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。
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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。
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
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(www.php.cn)!

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