select isnull(m.yue , n.yue) yue , m.cnta [A所有记录总数] , n.cntb [B所有记录总数] from
(
  select month([datetime]) yue, count(*) cnta from A group by month([datetime])
) m
full join
(
  select month([datemonth]) yue, count(*) cntb from B group by month([datemonth])
) n
on m.yue = n.yue

SELECT
    MON.month,
    A所有记录总数 = A.cnt,
    B表的记录数 = B.cnt
FROM(
    SELECT month = N'一月', months = 1 UNION ALL
    SELECT month = N'二月', months = 2 UNION ALL
    SELECT month = N'三月', months = 3 UNION ALL
    SELECT month = N'四月', months = 4 UNION ALL
    SELECT month = N'五月', months = 5 UNION ALL
    SELECT month = N'六月', months = 6 UNION ALL
    SELECT month = N'七月', months = 7 UNION ALL
    SELECT month = N'八月', months = 8 UNION ALL
    SELECT month = N'九月', months = 9 UNION ALL
    SELECT month = N'十月', months = 10 UNION ALL
    SELECT month = N'十一月', months = 11 UNION ALL
    SELECT month = N'十二月', months = 12
)MON
    LEFT JOIN(
        SELECT
            months = DATEPART(Month, datetime),
            cnt = COUNT(*)
        FROM A
        GROUP BY DATEPART(Month, datetime)
    )A
        ON MON.months = A.months
    LEFT JOIN(
        SELECT
            months = datemonth,
            cnt = COUNT(*)
        FROM B
        GROUP BY datemonth
    )B
        ON MON.months = B.months