ITPEC IP April 2021 Question 29
In a relational database, when (A∪B) and (A∩B) are performed on table A and table B shown below, which of the following is an appropriate combination of the resulting two tables among the three tables P, Q, and R? Here, (A∪B) shows the union of A and B, and (A∩B) shows the intersection of A and B.
A
B
P
Q
R
Explanation
Union is R, intersection is P — a union takes every row in either table, an intersection only the rows in both.
- •A holds P001, P003, P007, P012, P019; B holds P003, P007, P020
- •Union: all five rows of A plus the row of B not already in A, P020, each row once: six rows, table R
- •Intersection: the rows in both, P003 PC and P007 USB hub: table P
- •Q (P001, P012, P019) is the difference A − B, the rows in A but not in B
Answer (c)
Why not others:
- •(a) — swaps the two results: P is the intersection and R the union
- •(b) — Q is the difference A − B, not the union, and R is the union, not the intersection
- •(d) — the union is right, but Q is the difference A − B: it holds the rows A does not share with B, the opposite of the intersection
Key rule: Union, intersection and difference work on tables with the same columns; a row present in both tables appears once in the union.
Databases · AI-written, may contain errors
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