GATE Computer Science (CS) 2018 Shift 1 Solved Paper

Question : 60 of 65

Marks: +1, -0

Consider the relations r ( A ,B ) and s (B , C ) ,where sB. is a primary key and r.B is a foreign key referencing s.B. . Consider the query

Q: r ⊲ ⊳(s))

Let LOJ denote the natural left outer-join operation. Assume that r and s contain no null values.Which one of the following queries is NOT equivalent to Q?

$σ_{B<5}(r ⊲ ⊳$
$σ_{B<5}(r {LOJ} s)$
$r LOJ(σ_{B<5}(s))$
$σ_{B<5}(r) {LOJ} s$

Validate

Solution:

Given query :

r ⊲ ⊳(s)

A	B	C
$a_1$	2	$c_1$
$a_2$	4	$c_1$
$a_4$	4	$c_1$

σ_{B, s} r ⊲ ⊳(s)

A	B	C
$a_1$	2	$c_1$
$a_2$	4	$c_1$
$a_3$	4	$c_1$

σ_{B, s} r ⊲ ⊳

A	B	C
$a_1$	2	$c_1$
$a_2$	4	$c_1$
$a_3$	4	$c_1$

σ_{B, s} r ⊲ ⊳(s))

A	B	C
$a_1$	2	$c_1$
$a_2$	4	$c_1$
$a_3$	4	$c_1$
$a_4$	6	null
$a_5$	6	null

σ_{B < s} ( r )⊲ ⊳

A	B	C
$a_1$	2	$c_1$
$a_2$	4	$c_1$
$a_4$	4	$c_1$

Option “c” query result not equal to given query.

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