#DBMS

Question

what will be the correct answer?

Answer 1

Select Count (*) from T where B = 2 , this query returns false means returns 0 .

So the inner query of EXISTS returns a value 0 , it considered as true to EXISTS . Hence The main query " Select * from T where EXISTS " have true inner query so it returns  all the rows which is value 3 .

That means EXISTS selects all 3 rows as inner query returns true .

Answer 2

count value =0

exists gets a value 0(but not NULL) it got a value hence it becomes true.

hence 3 rows will be selected

Answer 3

if inner query is like this......(SELECT A from T where B = 2)

then in this case answer would be  "0" bcoz here inner query will return 'NULL' and it will be taken as false for EXISTS....