how to sumup alias columns in case statement in SQL : SELECT E.EMPNO,E.DEPTNO,E.SAL,E.COMM,
COALESCE(SAL,0)+COALESCE(COMM,0) AS T1,
CASE
WHEN E.DEPTNO=10 THEN COALESCE(E.SAL,0)+COALESCE(E.COMM,0)
WHEN D.DNAME='ACCOUNTING' THEN COALESCE(E.SAL,0)-COALESCE(E.COMM,0)
ELSE 0 END AS T,
sum(T+T1) as T2

FROM EMP E LEFT OUTER JOIN DEPT D ON E.DEPTNO=D.DEPTNO;


getting below error
ORA-00904: "T1": invalid identifier
00904. 00000 -  "%s: invalid identifier"
*Cause:    
*Action:
Error at Line: 7 Column: 7