Specified delete rule not allowed with existing trigger.
说明：指定删除不与现有触发器允许的规则。
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。
If a DELETE rule is not explicitly defined, the on DELETE no ACTION definition is used by default.
如果没有明确定义DELETE规则，将默认使用ON DELETE NO ACTION定义。
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