select a.month, nvl(b.amount,0) amount

--decode(b.amount,'',0)

from (select to_char(sysdate, 'yyyy') || lpad(level, 2, 0) month

from dual

connect by level < 13) a,

(select to_char(((sum(t.txn_amt) + sum(t.coupon_amt)) / 100),

'FM999,999,999,990.90') amount,

substr(t.inst_date, 0, 6) month

from t_log_acct_auth t

where t.confirm_flag = 'Y'

and t.cancel_flag = 'N'

and t.return_flag = 'N'

and substr(t.inst_date, 0, 4) = to_char(sysdate, 'yyyy')

group by substr(t.inst_date, 0, 6)) b

where a.month = b.month(+) order by a.month