Search notes:

ORA-22818: subquery expressions not allowed here

create table tq84_ora_22818_test (a number);

begin
   insert into  tq84_ora_22818_test values (1   );
   insert into  tq84_ora_22818_test values (2   );
   insert into  tq84_ora_22818_test values (null);
end;
/

The following statement throws ORA-00937: not a single-group group function:

select
   count(a)                                               cnt_a,
  (select count(*) from tq84_ora_22818_test where a = 1)  cnt_a_1
from
   tq84_ora_22818_test;

The following statement throws ORA-22818: subquery expressions not allowed here

select
   count(a)                                               cnt_a,
  (select count(*) from tq84_ora_22818_test where a = 1)  cnt_a_1
from
   tq84_ora_22818_test
group by
  (select count(*) from tq84_ora_22818_test where a = 1);

drop table tq84_ora_22818_test;

Subqueries are not allowed to provide the value for a default column:

create table tq84_p (
   id  number primary key,
   cnt number  default (select count(*) from tq84_p)
);

ORA-22818: subquery expressions not allowed here

See also