使用内部联接，并在联接表之前使用GROUP BY *。

示例

create table positif ( trx, nett )
as 
select 3, 2147600 from dual union all
select 3, 2068300 from dual union all
select 4, 50000   from dual union all
select 5, 100000  from dual ;

create table negatif ( trx, nett )
as
select 3, -1073800 from dual union all
select 3, -1073800 from dual union all
select 3, -2068300 from dual union all
select 4, -20000   from dual union all
select 5, -100000  from dual ;

查询

select P.trx, P.sum_ totalpos, N.sum_ totalneg
from 
  ( select trx, sum( nett ) sum_ from positif group by trx ) P
  join
  ( select trx, sum( nett ) sum_ from negatif group by trx ) N
  on P.trx = N.trx 
order by 1
;

-- result
TRX TOTALPOS    TOTALNEG
3   4215900     -4215900
4   50000       -20000
5   100000      -100000

DBfiddle
完全联接的主要问题是返回的行太多。例如，尝试只联接TRX列-〉您将得到2乘以3（6）行。您需要的是：每个TRX值占一行。（因此，在联接之前进行分组。）
行太多...

select P.trx
from positif P full join negatif N on P.trx = N.trx ;

TRX
3
3
3
3
3
3
4
5

备选方案：您也可以使用UNION ALL，然后使用GROUP BY（和sum），例如
所有接头

select trx, nett as pos, null as neg from positif
union all
select trx, null, nett from negatif ;

-- result
TRX POS     NEG
3   2147600 null
3   2068300 null
4   50000   null
5   100000  null
3   null    -1073800
3   null    -1073800
3   null    -2068300
4   null    -20000
5   null    -100000

GROUP BY和SUM

select trx 
, sum ( pos ) totalpos, sum( neg ) totalneg 
from (
  select trx, nett as pos, null as neg from positif
  union all
  select trx, null, nett from negatif 
) 
group by trx 
order by 1 ;

-- result
TRX TOTALPOS    TOTALNEG
3   4215900 -4215900
4   50000   -20000
5   100000  -100000