Is there any way in TSQL to use variable/Alias in
Select First_Name as FN from PERSON
Where FN like 'D%'
This is too long for a comment.
As many similar questions point out, the answer is "no". The solution is typically a CTE or subquery. This "limitation" is a property of the SQL language, not of a particular database.
SQL Server does have a third method (which is why I am answering). This is more appropriate for a complex expression. Your example isn't a great example, but you can use
select p2.fn from person p outer apply (select p.First_Name as FN) p2 where p2.fn like 'D%'