Mysql中的Cascade，NO ACTION，Restrict，SET NULL_MySQL
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Mysql中的Cascade，NO ACTION，Restrict，SET NULL
外键约束对子表的含义:
如果在父表中找不到候选键,则不允许在子表上进行insert/update
外键约束对父表的含义:
在父表上进行update/delete以更新或删除在子表中有一条或多条对应匹配行的候选键时,父表的行为取决于：在定义子表的外键时指定的on update/on delete子句, InnoDB支持５种方式, 分列如下 ：
cascade方式
在父表上update/delete记录时，同步update/delete掉子表的匹配记录
On delete cascade从mysql3.23.50开始可用; on update cascade从mysql4.0.8开始可用
set null方式
在父表上update/delete记录时，将子表上匹配记录的列设为null
要注意子表的外键列不能为not null
On delete set null从mysql3.23.50开始可用; on update set null从mysql4.0.8开始可用
No action方式
如果子表中有匹配的记录,则不允许对父表对应候选键进行update/delete操作
这个是ANSI SQL-92标准,从mysql4.0.8开始支持
Restrict方式
同no action, 都是立即检查外键约束
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