Database Management Systems | Set 10

Following questions have been asked in GATE CS 2005 exam.

1) Let r be a relation instance with schema R = (A, B, C, D). We define r1 = ‘select A,B,C from r’ and r2 = ‘select A, D from r’. Let s = r1 * r2 where * denotes natural join. Given that the decomposition of r into r1 and r2 is lossy, which one of the following is TRUE?
(a) s is subset of r
(b) r U s = r
(c) r is a subset of s
(d) r * s = s

Answer (c)
Consider the following example with lossy decomposition of r into r1 and r2. We can see that r is a subset of s.

Table r
 A      B      C      D
 1     10     100    1000    
 1     20     200    1000    
 1     20     200    1001 

Table r1
 A      B      C
 1     10     100 
 1     20     200 

Table r2
 A     D  
 1    1000  
 1    1001

Table s (natural join of r1 and r2)
 A      B      C      D
 1     10     100    1000    
 1     20     200    1000    
 1     10     100    1001 
 1     20     200    1001 

2) Let E1 and E2 be two entities in an E/R diagram with simple single-valued attributes. R1 and R2 are two relationships between E1 and E2, where R1 is one-to-many and R2 is many-to-many. R1 and R2 do not have any attributes of their own. What is the minimum number of tables required to represent this situation in the relational model?
(a) 2
(b) 3
(c) 4
(d) 5

Answer (b)
See for explanation.

3) Consider a relation scheme R = (A, B, C, D, E, H) on which the following functional dependencies hold: {A–>B, BC–> D, E–>C, D–>A}. What are the candidate keys of R?
(a) AE, BE
(b) AE, BE, DE

Answer (d)
A set of attributes S is candidate key of relation R if the closure of S is all attributes of R and there is no subset of S whose closure is all attributes of R.
Closure of AEH, i.e. AEH+ = {ABCDEH}
Closure of BEH, i.e. BEH+ = {ABCDEH}
Closure of DEH, i.e. DEH+ = {ABCDEH}

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