select sum(AMOUNT)
  from (select AMOUNT
          from T_ACC
         where ACC_ID = :1
            --and DATE = :2
        union all
        select ...
          from T_TRANS
         where DEBIT_ACC_ID = :1 or CREDIT_ACC_ID = :1
       )

with t(AMOUNT,
DEBIT_ACC_ID,
CREDIT_ACC_ID) as
 (select 100, 1, null
    from dual
  union all
  select 200, null, 1
    from dual
  union all
  select 300, 2, null
    from dual)

select sum(case
             when DEBIT_ACC_ID = 1 then
              -AMOUNT
             when CREDIT_ACC_ID = 1 then
              AMOUNT
             else
              0
           end) BALANCE
  from t
 where DEBIT_ACC_ID = 1
    or CREDIT_ACC_ID = 1