Oracle 18c

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create table clob_tab (col1) as
select to_clob('27,88,100,25,26') from dual;
1 rows affected

create table abc (col2) as
select * from table(sys.odcinumberlist(21,22,23,24,25,26,27,28,29,30));
10 rows affected

with cte (n) as (
  select to_number(regexp_substr(col1, '(.*?)(,|$)', 1, level, null, 1))
  from clob_tab
  connect by level < regexp_count(col1, '(.*?)(,|$)')
)
select * from cte;

N

27

88

100

25

26

with cte (n) as (
  select to_number(regexp_substr(col1, '(.*?)(,|$)', 1, level, null, 1))
  from clob_tab
  connect by level < regexp_count(col1, '(.*?)(,|$)')
)
select *
from abc
where col2 not in (
  select n from cte
);

COL2

29

22

21

23

28

30

24

select json_array (col1 format json) from clob_tab;

JSON_ARRAY(COL1FORMATJSON)

[27,88,100,25,26]

select *
from abc
where col2 not in (
  select to_number(x.column_value)
  from clob_tab
  cross join xmltable (col1) x
);

COL2

21

22

23

24

28

29

30

select x.n
from clob_tab
cross join json_table (json_array(col1 format json), '$[*]' columns n number path '$') x

N

27

88

100

25

26

select *
from abc
where col2 not in (
  select x.n
  from clob_tab
  cross join json_table (json_array(col1 format json), '$[*]' columns n number path '$') x
);

COL2

29

22

21

23

28

30

24