SQL> CREATE TABLE t (x VARCHAR2(1), n NUMBER);
SQL> INSERT INTO t VALUES ('A',7);
SQL> INSERT INTO t VALUES ('A',5);
SQL> INSERT INTO t VALUES ('B',3);
SQL> INSERT INTO t VALUES ('B',9);
SQL> INSERT INTO t VALUES ('B',8);
SQL> INSERT INTO t VALUES ('C',1);
SQL> INSERT INTO t VALUES ('D',2);
SQL> INSERT INTO t VALUES ('D',3);
SQL> COMMIT;
  
Commit complete.
  
SQL> SELECT NVL(SUM(DECODE(x,'A',n,'C',n,<font color=red>0</font>)),<font color=red>0</font>) FROM t;
  
NVL(SUM(DECODE(X,'A',N,'C',N,<font color=red>0</font>)),<font color=red>0</font>)
-----------------------------------
                                 13
  
SQL>