50. In the relational pattern R<U>, XYZ is a subset of U. Which of the following descriptions of multi-valued dependencies is correct is ().
A. If X→→Y is a trivial multi-valued dependency, then UXY is an empty set
B. If X→→Y is a trivial multi-valued dependency, then Y is a subset of X
C. If X→→Y and Y→→Z, then X→→Z
D. If X→→Y, then X→Y
This question examines the basic knowledge of relational database standardization theory.
Let R(U) be a relational pattern on an attribute set U, X, Y, and Z are subsets of U, and Z=UXY, multi-valued dependency X→→Y holds if and only if for any of R Each value of the relation r, r on (X, Z) corresponds to a set of Y values, this set of values is only determined by the X value and has nothing to do with the Z value.
If X→→Y, and Z=φ, then X→→Y is called trivial multi-valued dependence.
Multi-valued dependencies have the following properties
Symmetry: If X→→Y, then X→→Z, where Z=U-X-Y.
Transitivity: If X→→Y, Y→→Z, then X→→Z－Y.
Functional dependence can be regarded as a special case of multi-valued dependence: if X→Y, then X→→Y, and vice versa.
If X→→Y, X→→Z, then X→→YZ.
If X→→Y, X→→Z, then X→→Y∩Z.
If X→→Y, X→→Z, then X→→Y－Z, X→→Z－Y.
The answer is: A