egor90

select coalesce(   ( SUM(mc_rating.name) / COUNT(mc_rating.name) ), 0::integer ) as rating, ...

Но не понимаю, если rating не null, то будет 2 колонки, а в противном случае одна?

Попробуйте подзапрос:

with _rating as ( select
    m_users.id,
    coalesce(   ( SUM(mc_rating.name) / COUNT(mc_rating.name) ), 0::integer ) as rating ....
)
select * from _rating r where r.rating > 0