Why doesn't count(extract stack) and how can I replace it?

Question

SQL

you need to get the number of identical months

select count(extract(month from birth_date)) as pupils
from pupil
group by pupils
having count(*) > 1

Answer 1

In theory, everything is solved here by the correct grouping:

select  pupils, extract(month from birth_date), count(1) as pupils
   from pupil
 group by pupils, extract(month from birth_date)
having count(1) > 1