SQL Server 2017

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create table Table1 (firstdate date, lastdate date );
insert into Table1 values ('2019/10/02', '2019/10/03')
1 rows affected

SELECT T.firstdate
       , T.lastdate 
       , datediff(day, T.firstdate , T.lastdate) as Alias1
       , case when (datediff(day, T.firstdate , T.lastdate )) > 15 
            then 'a' 
            else 'b' 
         end as Alias2
FROM Table1 T;


firstdate
lastdate
Alias1
Alias2


2019-10-02
2019-10-03
1
b