【多选题】【消耗次数：1】
被参照表更新时，参照表可以进行的完整性违约处理规则可以是________
①
级联更新
②
置空值
③
拒绝操作
④
保持不变
参考答案：
复制
纠错
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在Visual FoxPro中,参照完整性规则包括更新规则、删除规则与[填空1] 规则。
【简答题】
参照完整性规则是对[填空]的约束。
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