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is option C the answer?

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yes. Plz explain

Answer 1

They are asking equal number of minimum possible tuples retrieved.

Here the word minimum possible is very important.

Lets take an example:

                                                        Table P

Col1	Col2
1	2
3	4

                                                         Table Q

Col1	Col2
1	2

Note: I have assumed different number of tuples in each table as they have given

Now, query 1 and query 2 give the equal number of minimum tuples though they may not be same.

Query 3 will give 2 tuples which eliminates this option.

Lets turn to query 4.

I break the entire query in 3 parts.

Q₄₁  exists  (  Q₄₂  

                      except

                      Q₄₃ )

Now, lets take the tables for this entire query.

                                      Table Q

B1	B2
1	2

                                        Table P

B1	B2	A1	A2
1	2	1	2
3	4	3	4

Lets move to execution of the entire query.

Now, Q₄₁ selects the tuple (1,2,1,2) from table P.

When Q₄₃ is executed, only the tuple (1,2) will be returned.

$\therefore$ Q₄₂ except Q₄₃ will return 0 tuples.

$\therefore$ Q₄₁ will not select the tuple (1,2,1,2) as exists(0 tuples) is false.

Now, Q₄₁ selects the tuple (3,4,3,4) from table P.

When Q₄₃ is executed, only the tuple (3 ,4) will be returned.

$\therefore$ Q₄₂ except Q₄₃ will return tuple (1, 2).

$\therefore$ Q₄₁ will select the tuple (3,4,3,4) as exists( 1 tuple ) is true.

Thus, execution of the entire query returned 1 tuple which matches with output of 1st and 2nd query.

Hence, option C.

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This is just a possibilty.  Its possible that query 4 returns 0 tuples if both the tables, P and Q,  contain same tuples. So, as per this, even option A is correct.

For the 4th query, they should give that both tables contain different tuples.

Comments

thnx.