Adityaanand,

When referring to the code

select salary*months as earnings, count(*) from employee
group by 1

"group by 1" is simply stating that you want to group by the data in Column 1.

Since the columns are created in the order they are declared, column 1 or just "1" in this case, is "earnings" since it is declared first.

Therefore in this situation: "Group by 1" is just shorthand for "Group by Earnings".

Cheers.