Doubt

Question

R(A , b, c) with n tuples, S(A , D, E). How many max tuples in R natural join S? {A is key for both}

• A. $\min (m,n)$
• B. $\max(m,n)$
• C. $m*n$
• D. $m+n$

Comments

here i guess key mean FK only thats why we are joining na? if both table are independent and dont have any parent key relationship then what is the adv. of joining

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I think we can join even there is no primary key foreign key relationship . if two tables have same column type we can join .

@habib check this

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Answer is (A).

Natural join requires the attributes to be of the same name and type.

Generally we join 2 tables with a PK-FK relation because it is only logical to join such tables....otherwise we may end up with spurious tuples....but theoretically there is no bound or condition stating a PK-FK relation is necessary.

Answer 1

Relation R(A , b, c)

A	B	C
1	ani	hi
2	cse	you
3	mca	me

Relation S(A , D, E).

A	D	E
1	ani	hi
2	cse	you

Performing Natural Join: $\pi (\sigma( R\times S))$ 

A	B	C	A	D	E
1	ani	hi	1	ani	hi
1	ani	hi	2	cse	you
2	cse	you	1	ani	hi
2	cse	you	2	cse	you
3	mca	me	1	ani	hi
3	mca	me	1	cse	you

Result:

A	B	C	A	D	E
1	ani	hi	1	ani	hi
2	cse	you	2	cse	you

So only 2 rows comes as result satisfy by option A min(R,S) = S here.

Comments

thanks much appreciated :)

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Cool...

Answer 2

Actually the question is asking about the max tuples in R natural join S.
so R natural join S  will only give max tuple when the

*    elements of the A(key of R)  are  the subset of the elements of the A ( key of S)
                              OR
*    elements of the A(key of S)  are  the subset of the elements of the A  ( key of R)
if we will take either of the above scenario we will get the result i.e
  (A)        min (m,n)