找出 2 个日期之间的月数

select 
  (age('2012-11-30 00:00:00'::timestamp, '2012-10-31 00:00:00'::timestamp)),
  (age('2012-12-31 00:00:00'::timestamp, '2012-10-31 00:00:00'::timestamp)),
  (age('2013-01-31 00:00:00'::timestamp, '2012-10-31 00:00:00'::timestamp)),
  (age('2013-02-28 00:00:00'::timestamp, '2012-10-31 00:00:00'::timestamp))

给出以下结果：

  0 years 0 mons 30 days 0 hours 0 mins 0.00 secs
  0 years 2 mons 0 days 0 hours 0 mins 0.00 secs
  0 years 3 mons 0 days 0 hours 0 mins 0.00 secs
  0 years 3 mons 28 days 0 hours 0 mins 0.00 secs

但我想要下个月的定义，我该怎么做？

  0 years 1 mons 0 days 0 hours 0 mins 0.00 secs
  0 years 2 mons 0 days 0 hours 0 mins 0.00 secs
  0 years 3 mons 0 days 0 hours 0 mins 0.00 secs
  0 years 4 mons 0 days 0 hours 0 mins 0.00 secs

表达方式

age('2012-11-30 00:00:00'::timestamp, '2012-10-31 00:00:00'::timestamp) 

gives 30 days。我们在期待1 month因为这两个值都指向该月的最后几天。如果我们在值中添加 1 天，我们将得到下个月的第一天，

age('2012-12-01 00:00:00'::timestamp, '2012-11-01 00:00:00'::timestamp)

预计会给我们 1 个月的时间。因此，让我们检查是否有该月的最后两天，在这种情况下返回接下来几天的年龄间隔。在其他情况下，我们将返回原始值的年龄区间：

create or replace function age_m (t1 timestamp, t2 timestamp)
returns interval language plpgsql immutable
as $$
declare
    _t1 timestamp = t1+ interval '1 day';
    _t2 timestamp = t2+ interval '1 day';
begin
    if extract(day from _t1) = 1 and extract(day from _t2) = 1 then
        return age(_t1, _t2);
    else
        return age(t1, t2);
    end if;
end $$;

一些例子：

with my_table(date1, date2) as (
values
    ('2012-11-30 00:00:00'::timestamp, '2012-10-31 00:00:00'::timestamp),
    ('2012-12-31 00:00:00'::timestamp, '2012-10-31 00:00:00'::timestamp),
    ('2013-01-31 00:00:00'::timestamp, '2012-10-31 00:00:00'::timestamp),
    ('2013-02-28 00:00:00'::timestamp, '2012-10-31 00:00:00'::timestamp)
)

select *, age(date1, date2), age_m(date1, date2)
from my_table

        date1        |        date2        |      age       | age_m  
---------------------+---------------------+----------------+--------
 2012-11-30 00:00:00 | 2012-10-31 00:00:00 | 30 days        | 1 mon
 2012-12-31 00:00:00 | 2012-10-31 00:00:00 | 2 mons         | 2 mons
 2013-01-31 00:00:00 | 2012-10-31 00:00:00 | 3 mons         | 3 mons
 2013-02-28 00:00:00 | 2012-10-31 00:00:00 | 3 mons 28 days | 4 mons
(4 rows)