DECODE is not working in WHERE clause when subquery returns more rows

Question

Hi Gurus,
I want to write a query on CCENTERS table(Script given below) and expect the following result:
1. When I pass a value of 0 for ID, It returns all the rows given in the table.
2. When I pass a value other than 0, It returns the row for the given value as well as all its child records.
CCENTER has parent-child relationship in ID and BASE column. I am using a query with DECODE function. but DECODE function in WHERE clause is not capable of handling sub-query with multiple rows.
VARIABLE ParaCCenter NUMBER
BEGIN
:paraccenter:=0;
END;
CREATE TABLE ccenters
(id NUMBER,
name VARCHAR2(20),
base number);
INSERT INTO ccenters VALUES(1,'NUST',null);
INSERT INTO ccenters VALUES(2,'SEECS',1);
INSERT INTO ccenters VALUES(3,'NBS',1);
commit;
SELECT * FROM ccenters
WHERE id IN DECODE(:ParaCCenter, 0, id,
(SELECT id FROM ccenters
START WITH base=:ParaCCenter
CONNECT BY PRIOR id = base
UNION
SELECT :ParaCCenter FROM dual
BEGIN
:paraCCenter:=1;
END;
SELECT * FROM ccenters
WHERE id IN DECODE(:ParaCCenter, 0, id,
(SELECT id FROM ccenters
START WITH base=:ParaCCenter
CONNECT BY PRIOR id = base
UNION
SELECT :ParaCCenter FROM dual))
The result is
(SELECT id FROM ccenters
ERROR at line 3:
ORA-01427: single-row subquery returns more than one row
How this query can be rewritten for the given functionality. Any response will be highly appreciated.
Thanks

Answer 1

And if you want to use DECODE:
SQL> BEGIN
  2  :paraccenter:=0;
  3  END;
  4  /
PL/SQL procedure successfully completed.
SQL> select  *
  2    from  ccenters
  3    where :paraccenter = decode(:paraccenter,0,0,id)
  4  /
        ID NAME                       BASE
         1 NUST
         2 SEECS                         1
         3 NBS                           1
SQL> BEGIN
  2  :paraccenter:=2;
  3  END;
  4  /
PL/SQL procedure successfully completed.
SQL> select  *
  2    from  ccenters
  3    where :paraccenter = decode(:paraccenter,0,0,id)
  4  /
        ID NAME                       BASE
         2 SEECS                         1
SQL> SY.