Question: The PLSQL code below will cause ORA-00934: group function is not allowed here error message
select case when count(...) then ... else ... end from some_table
But it will be no problem if in reverse order, e.g.
select sum(case when ... then 1 else 0 end) from some_table
You can count these situations you want by summing these numbers which being one while fitting the right condition.
Likely, you can also count how many kinds of situation by counting with differenct conditions.
select count(case when a>10 and b<5 then 'a10b5' when a+b=5 then 'ab5' else 'others' end) from some_table