find users with no activity

gistfile1.txt

-- members
create table members (id integer, age integer);

insert into members (id,age) values
(1,10) ,(2,20) ,(3,30) ,(4,25) ,(5,28) ,(6,37) ,(7,38) ,(8,68) ,(9,9)

-- posts
create table posts (id integer, user_id integer, title varchar(32))

insert into posts (id,user_id,title) values
(1,1,'my blog'),
(2,3,'my awesome blog'),
(1,1,'my mediocre blog')

-- how many members have not posted anything
select count(m.id) 
from members m 
left outer join posts p on (m.id = p.user_id)
where p.user_id IS NULL