Virtual Gate Test Series: Databases - Sql query

Question

Consider the following instance R.

A	1	2	3	4
B	Null	1	2	2

The number of tuples returned by the following SQL query is _____________.
> select * from R as R1
   where not exists (select * from R)  where B = R1.A                          

Comments

0 rows, Comparing with NULL will give undefined result.

---

But answer given in the Virtual Gate Test Series is 2

---

2 is correct output is
2,4
2,3

---

Please explain.

---

2,3 and 2,4 is this tuple exist ?

I have considered this table by rotating it  :p

A	B
1	null
2	1
3	2
4	2

Now if we check B = R1.A all tuples will present in it then NOT exist never goes true

And mistakenly I considered A = R1.B then also comparing with null gives 0 rows.

---

yes they exist i wrote (B,A) ..

---

any comparision with NULL will give false..

for A=1 B=null will give False but 1 will give true and not exists lead to False....similerly for 2,but for 3,4 ...null,1,2,2 all will result false therefor result no touple...and not exits true....

therefor ...it will return two touple....

---

Any comparison with NULL will give undefined or false? I think undefined

---

yes undefined ....

let comparison is like this....

any Number =NULL (here we r not sure about the value of NULL therefor False)

Number>NULL(again we r not sure about the value of NULL therefor False)

Number<NULL(again we r not sure about the value of NULL therefor False).....

therefor we say that the comparison with NULL gives undefine ...

---

Then NOT exist (undefined) will not return any rows

---

I think the answer should be 2 as the two tuples with A as 3 and 4 should be returned as they do not have an equivalent value in B.

Answer 1

there is a mistake in parathezisation...it will be 2