In a conceptual database schema, there is a superclass entity T. T has an attrib
ID: 3859807 • Letter: I
Question
In a conceptual database schema, there is a superclass entity T. T has an attribute-defined cluster of partial and disjoint subclasses based on the attribute P. There are two subclasses, U1 and U2, in this cluster.
In a valid state of of this schema, entity instances e1, e2, and e3 are already in T. This state would still be valid if it also includes __________.
(a) entity instance e4 which is also in T but not U1 nor in U2
(b) entity instance e4 which is also in U1 but not in T nor in U2
(c) entity instance e4 which is also in U2 but not in T nor in U1
(d) entity instance e4 which is also in U1 and U2 but not in T
(e) None of (a) through (d) can be a valid state of this design.
(a) entity instance e4 which is also in T but not U1 nor in U2
(b) entity instance e4 which is also in U1 but not in T nor in U2
(c) entity instance e4 which is also in U2 but not in T nor in U1
(d) entity instance e4 which is also in U1 and U2 but not in T
(e) None of (a) through (d) can be a valid state of this design.
Explanation / Answer
Ans: (A) Entity instance e4 which also in T, but not U1 nor in U2.
Why means: we have two subtypes those partial and disjoint
Partial:T will have relation with condition like "Could be U1 or U2 or Neither "
Disjoint: Can either U1 or U2 but not both.
if we declare e4 in U1 or U2 Or in both also we can't use efficiently, So we orefer to use in T.