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+17 votes

Consider the join of a relation $R$ with a relation $S$. If $R$ has $m$ tuples and $S$ has $n$ tuples then the maximum and minimum sizes of the join respectively are

  1. $m+n$ and $0$
  2. $mn$ and $0$
  3. $m+n$ and $|m-n|$
  4. $mn$ and $m+n$


asked in Databases by Veteran (69k points)
edited by | 3.9k views
ans plz

1 Answer

+30 votes
Best answer


Case 1: if there is a common attribute between R and S, and every row of $r$ matches with the each row of $s$- i.e., it means, the join attribute has the same value in all the rows of both $r$ and $s$,
Case 2: If there is no common attribute between R and S.

$0$ There is a common attribute between R and S and nothing matches- the join attribute in $r$ and $s$ have no common value.

answered by Boss (8.6k points)
mn occurs during cartesian product of two relation ....

and 0 occurs during natural join of two realtion(In best case )....
Join means Natural Join only.
Natural Join gives result as cartesian product if no attribute is matching in both relations.
eg- $R(A,B)$ and $S(C,D)$

Natural Join gives result as empty relation  if  at least one of the attribute matches but value does not matches.
$\begin{tabular}{ |c|c| }

\hline \multicolumn{2}{|c|}{R} \\

\hline A & B \\

\hline a1 & b1 \\

a2 & b2

\\ a3 & b3 \\



$\begin{tabular}{ |c|c| }

\hline \multicolumn{2}{|c|}{S} \\

\hline B & C \\

\hline b3 & c1 \\

b4 & c2 \\

b5 & c3 \\

\hline \end{tabular}$

In this case $R \Join S$ is empty.
Thanks @Sachin it  more clear now.

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