GATE | GATE-CS-2015 (Set 2) | Question 65

Consider two relations R1(A, B) with the tuples (1, 5), (3, 7) and R1(A, C) = (1, 7), (4, 9). Assume that R(A,B,C) is the full natural outer join of R1 and R2. Consider the following tuples of the form (A,B,C)

  a = (1, 5, null),
  b = (1, null, 7), 
  c = (3, null, 9), 
  d = (4, 7, null), 
  e = (1, 5, 7), 
  f = (3, 7, null), 
  g = (4, null, 9). 

Which one of the following statements is correct?
(A) R contains a, b, e, f, g but not c, d
(B) R contains a, b, c, d, e, f, g
(C) R contains e, f, g but not a, b
(D) R contains e but not f, g


Answer: (C)

Explanation:

Below is R1 
  A   |   B
 -----------
  1   |   5
  3   |   7


Below is R2
  A   |   C
 -----------
  1   |   7
  4   |   9 


Full outer join of above two is 

  A   |   B   |   C
 -------------------
  1   |   5   |   7
  3   |   7   |  NULL
  4   |  NULL |   9

So the full outer join contains e = (1, 5, 7), f = (3, 7, null), g = (4, null, 9).


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