How to find the average value?

Question

PostgreSQL

select subject, role
from tetrika.lessons as tl, tetrika.users as tu
where tu.role = 'tutor'
group by role, tutor_id, tu.id, subject
having tl.tutor_id = tu.id

I need to find the average value for the subject column, I was thinking of doing this:

select (sum(subject) :: int / count(subject)) as subject_avg, subject, role
from tetrika.lessons as tl, tetrika.users as tu
where tu.role = 'tutor'
group by role, tutor_id, tu.id, subject
having tl.tutor_id = tu.id

But it did not work out very well to cast the character varying type to int

spoiler