48. In the relational model R<U,D>, D is the set of R's functional dependency and multi-valued dependency. Decompose R into two relational patterns R1<U1,D1> and R2<U2,D2>, then the error in the following statement is ().
A. If U1∩U2→U1-U2 is true, then this decomposition has lossless connectivity
B. If U1∩U2→→U1-U2 is true, then this decomposition has lossless connectivity
C. If U1∩U2→U1-U2 does not hold, then this decomposition does not have lossless connectivity
D. If U1∩U2→→U1-U2 does not hold, then this decomposition does not have lossless connectivity
This question examines the basic knowledge of relational database standardization theory.
The necessary and sufficient condition for ρ to have a lossless connection is: U1⋂U2→U1-U2∈F+ or U1⋂U2→U2-U1∈F+.
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, then X→→Y, it means that the functional dependency is a special case of multi-valued dependency. When X→Y, there is a Y corresponding to each X.
According to the meaning of the question, it is necessary to examine the relationship between functional dependence and multi-value dependence on the basis of understanding the sufficient and necessary conditions for lossless connection.
A statement is correct
The statement B is wrong, because U1∩U2→→U1-U2 is established, which does not mean that U1∩U2→U1-U2 is established
C is correct
D is correct, because U1∩U2→→U1-U2 does not hold, obviously U1∩U2→U1-U2 does not hold either
The answer is: B