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1 vote
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A system where commutativity is considered different, in how many ways the join of n  relations r1,r2,…, rn can be taken?

1. (2n−2)!/(n−1)!
 
2. (2n−2)!/n!(n−1)!
 
3. (2n−1)!/n!(n−1)!
 
4. None
in Databases 213 views

2 Answers

0 votes
Option 1.
0 votes

for n=2 ...

1.r1 join r2 and r2 join r1  ....option 2 and 3 are eliminated ....

now for n=3(it is 12) ...  r1Jr2Jr3{can be arranged in 6 way and for each arrangement(2way for associativity)...

similarly for n=4 (i.e 120).. i think 1st is the option ??????

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