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In SQL, relations can contain null values, and comparisons with null values are treated as unknown. Suppose all comparisons with a null value are treated as false. Which of the following pairs is not equivalent?

  1. x = 5 $\quad$ not (not (x = 5)) 
  2. x = 5 $\quad$ x > 4 and x < 6, where x is an integer
  3. x ≠ 5 $\quad$ not (x = 5)
  4. none of the above

 

asked in Databases by Veteran (68.8k points)
edited by | 2.6k views

3 Answers

+32 votes
Best answer

answer = option C

answered by Veteran (30.5k points)
selected by
@amarVashishth can oyu please explain me the options given, Like i am not able relate between null and x=5. Where does comparision with null comes in picture?

@Aayushi, I am trying to explain option c.

Suppose all comparisons with a null value are treated as false.

now see, LHS = x !=5,  lets take x=null, => (null) !=5                  => false.

              RHS = not( x = 5) ,  => not ( null = 5)    =>> not (false) =>> true.

Now check above solution for other options..

Can u explain about all options and their correctness, means how are they correct
+24 votes
Answer: C

As (null) != 5 gives false.
But, not((null) = 5) gives not(false) gives true.
answered by Loyal (3.4k points)
edited by
option (b)
NULL=5   False
x>4 and x<6 where x is an integer
Please explain how this evaluates to false
in all the options, will be evaluating the pair of equations with same value of x(whether it is null or an integer) in option 'b' x is always an integer, so whatever value of x u put both x=5 and x>4 and x<6 will always give the same answer
Yes. THANKS
+7 votes
a.null = 5   => false ,    

 not(not(null=5))  = not(not(false)) = not(true) = false

b. null = 5 => false ,

null>4 and null <6 = False and false = false

c.null ≠ 5 => false

not(null= 5)= not (false) = true

∴ option C is not eqivivalent
answered by Junior (693 points)


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