According to me Best approach to solve such question is by taking example which includes all possible options.
In case one of option is given "none of above" in that case be also check whether final answer returns null , or multiple value.
This approach is time taking but gives correct result.
customers : A,B,C,D shops: P1,P2,P3
Buys
A |
P1 |
B |
P2 |
B |
P2 |
C |
P1 |
C |
P2 |
C |
P3 |
reason why only this table taken
first row :
Customers who buy from at least one shop. -> then A must be there, and D will not be there
second and third row
Customers who buy from at least two shops.-> then B will be there and A, D will not be there
4,5,6th
Customers who buy from all shops. -> then C will be there A, B, D will not be there.
No row for D:
Customers who do not buy buy anything at all. ->D will be there A, B, C will not be there
ΠCName((C×S)−B)
gives
A |
P2 |
A |
P3 |
B |
P1 |
D |
P1 |
D |
P2 |
D |
P3 |
I know projection will return just (A, B, D) , but for understanding purpose i have written like this
final answer
C