Can Anyone explain why NULL is used in this query?

Question

Also what will be the scenarios where this query is used

select * from TableA where exists  
    (select null from TableB where TableB.Col1=TableA.Col1)

Answer 1

As the query is in an EXISTS then you can return anything. It is not even evaluated.

In fact, you can replace the null with (1/0) and it will not even produce a divide by zero error.