Oracle 18c

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create table your_table (ID, PERIOD_1, PERIOD_2, PERIOD_3) as
select 0, 6, 5, 5 from dual union all
select 1, 120, 124, 85 from dual union all
select 2, 183, 159, 99 from dual union all
select 3, 60, 42, 27 from dual union all
select 4, 24, 19, 8 from dual union all
select 5, 1, null, null from dual
6 rows affected

select id, period_1, period_2, period_3,
  nvl(period_1, 0) + nvl(period_2, 0) + nvl(period_3, 0) as total,
  round(100 * period_1 / (nvl(period_1, 0) + nvl(period_2, 0) + nvl(period_3, 0)), 2) as period_1_perc,
  round(100 * period_2 / (nvl(period_1, 0) + nvl(period_2, 0) + nvl(period_3, 0)), 2) as period_2_perc,
  round(100 * period_3 / (nvl(period_1, 0) + nvl(period_2, 0) + nvl(period_3, 0)), 2) as period_3_perc
from your_table;


ID
PERIOD_1
PERIOD_2
PERIOD_3
TOTAL
PERIOD_1_PERC
PERIOD_2_PERC
PERIOD_3_PERC


0
6
5
5
16
37.5
31.25
31.25

1
120
124
85
329
36.47
37.69
25.84

2
183
159
99
441
41.5
36.05
22.45

3
60
42
27
129
46.51
32.56
20.93

4
24
19
8
51
47.06
37.25
15.69

5
1
null
null
1
100
null
null


select id, period_1, period_2, period_3,
  round(100 * period_1 / total, 2) as period_1_perc,
  round(100 * period_2 / total, 2) as period_2_perc,
  round(100 * period_3 / total, 2) as period_3_perc
from (
  select id, period_1, period_2, period_3,
    nvl(period_1, 0) + nvl(period_2, 0) + nvl(period_3, 0) as total
  from your_table
);


ID
PERIOD_1
PERIOD_2
PERIOD_3
PERIOD_1_PERC
PERIOD_2_PERC
PERIOD_3_PERC


0
6
5
5
37.5
31.25
31.25

1
120
124
85
36.47
37.69
25.84

2
183
159
99
41.5
36.05
22.45

3
60
42
27
46.51
32.56
20.93

4
24
19
8
47.06
37.25
15.69

5
1
null
null
100
null
null

ID	PERIOD_1	PERIOD_2	PERIOD_3	TOTAL	PERIOD_1_PERC	PERIOD_2_PERC	PERIOD_3_PERC
0	6	5	5	16	37.5	31.25	31.25
1	120	124	85	329	36.47	37.69	25.84
2	183	159	99	441	41.5	36.05	22.45
3	60	42	27	129	46.51	32.56	20.93
4	24	19	8	51	47.06	37.25	15.69
5	1	null	null	1	100	null	null