Database

Question

Consider 3 relation R(A, B, C) WITH 20 TUPLES  and S(A,D , E) With 30 tuples and T(D,F,G ) with 40 tuples.

What is the max no. of tuples in  natural joint  of R S T

Plz explain

Comments

Answer :2400?

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No.. Its 30.

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you forget to mark candidate key.

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If answer is 30 then D in S( A, D, E) should be a key ..

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@anirudh.. sorry i forgot... now check .. I have underlined the primary keys..

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Now ans should be 20 only , because R * S will give only 20 touples and (R*S)*T .. will again give maximum 20 touples.

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@Vijay, R join S will give max of 30 tuples bcause in S, there can be more than 1 tuple with same value for A.

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Yes, you are correct @Sushant. my bad  :p

Answer 1

COnsider the data values as follows:

R1                        R2                         R3

--------------------------------------------------------------------------

A                   A   D                           D

-------------------------------------------------------------------------

1                   1    1                            1

.                    .     .                             .

.                    .     .                             .                            

20                 20   20                           .

                     1     21                          .

                     .     .                             .

                     .     .                             .

                     10  30                           40

SO, R join S will give max of 30 tuples and that join with T will give max of 30 tuples only because T contains unique tuples.

Comments

I agree with this.A of S is referencing R.So every value of A which is there in S will also be there in R.Also a value in S will reference to only one value of A in R as A is a primary key in R.So

result of R join S will have 30 tuples.Now there are 30 tuples in natural join of R and S and 40 tuples in T.Now D attribute of R join S will reference to primary key D of T which contains 40 tuples.

So 30 tuples of R join S will match with atmost 30 tuples of T.

Hence maximum no. of tuples of R join S join T = 30