Correct answer? (SQL)

Question

I am reading the following question from a book.
If we are required to construct an equi join of 30 tables, how many join
conditions will be required?
a. 30
b. 20
c. 29
d. The number cannot be determined
I wonder what is the correct answer to the following question C or D?
The book says correct is C.
The theory says
"To join n tables together, you need a minimum of n-1 join conditions.
For example, to join four tables, a minimum of three joins is required.
This rule may not apply if your table has a concatenated primary key,
in which case more than one column is required to uniquely identify each row."
What should I answer if this question was in a 1Z0-051 test?
I would answer D. Is it wrong?

Answer 1

Hi,
As others have said, the question is vague.  What does it mean for 30 tables to be equijoined?  Does "a.col1=b.col1 AND a.col2=c.col2" count as 1 join condition or 2?  The best you can do is rule out the answers that don't make any sense, and guess at what the author wants from the remaining choices.
In this problem, the answerrs 20 and 30 don't make any sense.  No matter what assumptions or definitions you make, any reasoning that results in an answer of 20 could also wiork for 19, or 21, or 30, that is, "the number cannot be determined".
So the remaining choices are "29" and "the number cannot be determined".  I would guess they want "29", but that's just a guess.  As mentioned already, if you are joining N tables, and none of the joins are unconditional (that is, cross joins), then you need N-1 join conditions, counting compound conditions such as "a.col1=b.col1 AND a.col2=c.col2" as 1 condition.  (It doesn't matter if all, or any, of the conditions are equi-joins.)