select m.id month, IFNULL(b,0) num from
(
select 1 as id union select 2 union select 3 union select 4 union select 5 union select 6
union select 7 union select 8 union select 9 union select 10 union select 11 union select 12
) m
left join
(
SELECT
MONTH (create_time) a,
count(user_account_id) b
FROM
tb_activity_reg
WHERE
YEAR (create_time) = YEAR (NOW())
GROUP BY
MONTH (create_time)
) n
on m.id = n.a
ORDER BY month asc