What would be the result of this query

Question

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Can anyone please tell me the answer for this, with explaination. According to me it will give the customer who buy from atleast one store, but the answer given is customer who buy from all stores.

Answer 1

@print

You forget to add projection operator after (C*S)-B table .

That means ∏Cname (C*S-B ) = B,C ….

now C - ∏Cname (C*S-B ) = A , as C= (A,B,C )  ….

 

A means customer who buy from all shops. As

A   S1

A   S2

 

so A is related to shop 1 and shop 2 both ....

And shop table has only 2 entries s1 and s2 ....

Hence customer who buy from all shops ...

 

 

1. https://gateoverflow.in/19246/Tifr2010-b-33 

 

2. https://gateoverflow.in/25872/Tifr2013-b-19

 

 

Answer 2

For these kind of questions it is better to take examples to avoid confusion 

Comments

Right, solving by illustration is always a better choice.