Oracle 11g Release 2

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create table Table1 (firstdate date, lastdate date );
SELECT T.firstdate
       , T.lastdate 
       , (T.lastdate - T.firstdate) as ALIAS1
       , case when (T.lastdate - T.firstdate) > 15 then 'a' else 'b' end as ALIAS2 
FROM Table1 T;
select t.firstdate, t.lastdate, tt.alias1,
       (case when alias1 > 15 then 'a' else 'b' end) as alias2
from table1 t cross apply
     ( values (t.lastdate - t.firstdate) 
     ) tt(alias1);
ORA-00905: missing keyword

SELECT T.firstdate
       , T.lastdate 
       , T.lastdate - T.firstdate as ALIAS1
FROM Table1 T;
SELECT T.firstdate
       , T.lastdate 
       , T.lastdate - T.firstdate as ALIAS1
FROM Table1;
ORA-00904: "T"."FIRSTDATE": invalid identifier

SELECT T.firstdate
       , T.lastdate 
       , 3 - 1 as ALIAS1,
case when ALIAS1> 15 then 'a' else 'b' end as ALIAS2 
FROM Table1 T;
ORA-00904: "ALIAS1": invalid identifier