Y主键约束用来强制数据的()完整性。 A 域 B 实体 C 参照 D ABC都可以
时间: 2024-03-15 16:42:15 浏览: 27
Y主键约束用来强制数据的"实体"完整性。
在关系数据库中,实体完整性是指表中每一行数据都应该有一个唯一的标识。这个标识可以通过主键约束来实现。主键约束强制表中的某个列或列组合成为主键,并且保证该列或列组合中的每个值都是唯一的,即不存在重复的值。因此,主键约束可以用于实现实体完整性。因此,答案是B:“实体”。域完整性是指对表中某个列的取值进行限制,参照完整性是指确保在关系数据库中引用其他表的数据时,引用的数据是存在的。ABC选项都不正确。
相关问题
A’B(D’+C’)+ABC’D’化简逻辑函数
A’B(D’+C’)+ABC’D’可以通过以下步骤化简:
1. A’B(D’+C’)+ABC’D’ = A’B(D’+C’+CD’) + ABC’D’ (分配律)
2. A’B(D’+C’+CD’) + ABC’D’ = A’B(D’+C’) + ABC’D’ (吸收律)
3. A’B(D’+C’) + ABC’D’ = A’BD’ + A’BC’ + ABC’D’ (分配律)
4. A’BD’ + A’BC’ + ABC’D’ = A’BD’ + A’BC’ + ABDD’ + ABC’D’ (吸收律)
5. A’BD’ + A’BC’ + ABDD’ + ABC’D’ = A’BD’ + A’BC’ + ABC’D’ (恒等律)
6. A’BD’ + A’BC’ + ABC’D’ = A’BD’ + AB(C’+D’) (分配律)
7. A’BD’ + AB(C’+D’) = A’BD’ + ABC’ + ABD’ (分配律)
8. A’BD’ + ABC’ + ABD’ = A’BD’ + ABD’ + ABC’ (交换律)
9. A’BD’ + ABD’ + ABC’ = ABD’ + ABC’ (恒等律)
因此,A’B(D’+C’)+ABC’D’可以化简为ABD’+ABC’。
Prove If A → C and B → C and ABC → D, then A → D 数据库
To prove that A → D, we need to show that if A is true, then D must also be true.
Given that ABC → D, we know that if A and B and C are all true, then D must also be true.
We are also given that A → C and B → C, which means that if A is true, then C must also be true, and if B is true, then C must also be true.
Combining these two statements, if A is true, then B is also true (because if A were false, then we wouldn't have enough information to determine whether B is true or false). Therefore, if A and B and C are all true, then D must also be true (because of the ABC → D statement).
Therefore, we can conclude that A → D.
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