【多选题】【消耗次数：1】
对关系的描述正确的是（ ）
①
关系是一个集合
②
关系中一个属性可以由多个子属性组成
③
关系中不允许任意两个元组取值完全相同
④
关系中的元组次序可交换
参考答案：
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【简答题】
在关系模型中,“关系中不允许出现相同元组”的约束就是通过[填空1]实现的。
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