Questions on Super keys –

Question 1 :
 Relational schema R with n attribute A1, A2,…,An. How many super keys are possible with
  a) only candidate keys A1
 b) only Candidate Keys A1, A2
 c) only Candidate keys (A1A2), (A3A4)
 d) only candidate keys (A1A2), (A2,A3)
 e) only candidate keys A1,A2,A3 ∩∪⇒
Solution :
a) only Candidate Keys A1
  A1
  A1A2
  A1A3
  A1A2A3
  A1A2A3…..An

Common Attributes - {A2,A3,....An} = 2n-1 

So, Total Number of combinations of attributes that contains {A1} ⇒ 1 * 2{n-1} = 2{n-1}
b) only candidate keys A1,A2
(Table 1)
Super Keys possible with only candidate key A1
A1
A1A2
A1A3
A1A2A3
A1A2A3……An
⇒ 2n-1
(Table 2)
Super Keys possible with only candidate key A2
A2
A2A1
A2A3
A2A1A3
A2A1A3……An
⇒ 2n-1
Common Super Keys From Table 1 and Table 2  ⇒ {A3A4….An} = 2n-2 

So, Total Number of combinations of attributes that contains {A1}, {A2} or {A1,A2} ⇒ 3 * 2(n-2}
   3 * 2(n-1)  can be written as 
= (2+1) * 2(n-1)

= 2.2(n-1) + 1.2(n-1)
= 2(n-2) + 2(n-1)
So, From the above observation, if there are n attributes and m candidate keys, then the possible superkeys with m candidate keys will be -

2n-1 + 2n-2 + 2n-3 + ………… + 2n-m

Another Solution with the help of Venn Diagram :

Question on Super Keys with Solution 1b
c) only candidate keys (A1,A2), (A3,A4) 

Question on Super Keys with Solution 1c
d) Only Candidate keys (A1A2),(A2A3)

Question on Super Keys with Solution 1d
e) only candidate keys A1,A2,A3

Question on Super Keys with Solution 1e
Question 2 :
Relational scheme R with N attribute A1A2….An. If every attribute is the Candidate Key, then how many superkeys are possible?  {DRDO}
Solution :

Let R(ABC), so
 A
 B
 C
 AB
 BC
 AC
 ABC =7 =23-1
 It implies, 2n-1 Super Keys. Why 2n-1? because
Total Number of Subsets possible with n elements = 2n
But It includes Ø, However in case of SuperKey atleast 1 key is mandatory. As every attribute is candidate key, So,

2n-1
Question 3:
Consider a Relational Schema R with attributes A,B,C,D,E. 
How many super keys  are possible if
{A,B,C} : Candidate Keys
Solution :

3 Candidate Keys out of 5 attributes - resembling the state as m candidate keys out of n attributes. So, By applying the above formula, we get the following results -
A B C
25-1=16 25-2=8 25-3=4
=16+8+4=28

Verification :
A B C
AB
AC
AD
AE
BC
BD
BE
BA
CD
CE
CA
CB
ABC
ABD
ABE
ACD
ACE
ADE
BCD
BCE
BDE
ABC
ABD
ABE
CDE
ABC
ACD
ACE
BCE
BDE
ABCD
ABCE
ABDE
ACDE
BCDE
ABCD
ABCE
ABDE
ABCD
ABCE
ACDE
BCDE
ABCDE ABCDE ABCDE
16 8 4
The candidate keys which are in red colour, are duplicate. So, excluding those in the answer.
Question 4 : 
How many Superkeys are possible with only candidate keys A1,A2,A3,A4 if there are n attributes in a Relational Schema R? Explain the solution with the help of Venn Diagram.
Solution :

Question on Super Keys with Solution 4

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Database Keys in DBMS Types of Constraints in DBMS
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