The delete rule controls how delete operations performed against a parent table in an RI relationship are processed.
Delete规则控制对RI关系中父表执行的删除操作的处理方法。
Relationship delete Rules: a relationship's delete rule specifies what should happen if an attempt is made to delete the source object.
关系删除规则：关系的删除规则指定如果尝试删除源对象，应该发生什么。
As with the previous rules, the Delete rule is implicitly created as part of an RI constraint.
和上述规则一样，Delete规则是作为一个RI约束的一部分被隐式创建的。
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