Question

我在sql表中有两列是startdate和enddate

Startdate           Enddate
27-12-2015 22:30    03-01-2016 19:30
01-01-2016 12:45    09-02-2016 18:30

我想得到像

这样的结果表

Startdate           Enddate            Month     year   days
27-12-2015 22:30    03-01-2016 19:30   Dec       2015   5
27-12-2015 22:30    03-01-2016 19:30   Jan       2016   3
01-01-2016 12:45    09-02-2016 18:30   Jan       2016   31
01-01-2016 12:45    09-02-2016 18:30   Feb       2016   9

Answer 1

在这种特殊情况下，plpgsql函数可以提供比普通sql查询更好的性能。

create or replace function get_months(startdate date, enddate date)
returns table (mon text, year int, days int)
language plpgsql as $$
declare d date;
begin
    d:= date_trunc('month', startdate);
    while d < enddate loop
        mon:= to_char(d, 'Mon');
        year:= to_char(d, 'YYYY');
        days:= case 
            when d+ '1month'::interval > enddate then enddate- d+ 1
            when d < startdate then (d+ '1month'::interval)::date- startdate
            else (d+ '1month'::interval)::date- d
        end;
        return next;
        d:= d+ '1month'::interval;
    end loop;
end
$$;

测试：

with my_table(startdate, enddate) as (
    values
    ('2015-12-27 22:30', '2016-01-03 19:30'),
    ('2016-01-01 12:45', '2016-02-09 18:30')
)

select *
from my_table, 
lateral get_months(startdate::date, enddate::date)

    startdate     |     enddate      | mon | year | days 
------------------+------------------+-----+------+------
 2015-12-27 22:30 | 2016-01-03 19:30 | Dec | 2015 |    5
 2015-12-27 22:30 | 2016-01-03 19:30 | Jan | 2016 |    3
 2016-01-01 12:45 | 2016-02-09 18:30 | Jan | 2016 |   31
 2016-01-01 12:45 | 2016-02-09 18:30 | Feb | 2016 |    9
(4 rows)

Answer 2

粗略的解决方案是生成所有日子然后聚合（计数）它们。它有效，但记忆力很差。如果它不重要，这个解决方案肯定会奏效。如果性能至关重要，另一种方法是生成一个月系列，并在很多条件下进行日间差异。

SELECT
    dates.startdate::DATE,
    dates.enddate::DATE,
    to_char(days.s, 'Mon') AS mon,
    to_char(days.s, 'YYYY') AS yr,
    count(1) AS d
FROM dates
CROSS JOIN LATERAL (
    SELECT * FROM generate_series(dates.startdate, dates.enddate, INTERVAL '1 day') s
) days
GROUP BY 1, 2, 3, 4

在任何情况下，这是第二个变体，它会循环数月（更快，更难理解）：

SELECT
    dates.startdate::DATE,
    dates.enddate::DATE,
    to_char(months.startdate, 'Mon') AS mon,
    to_char(months.startdate, 'YYYY') AS yr,
    least(
        months.enddate::DATE - dates.startdate::DATE + 1, -- takes care of first month
        dates.enddate::DATE - months.startdate::DATE + 1, -- takes care of last month
        months.enddate::DATE - months.startdate::DATE + 1 -- takes care of full months from the middle of the intervals
    ) AS "days"
FROM dates
-- get months as first day in that month
CROSS JOIN LATERAL (
    SELECT * FROM generate_series(
        (to_char(dates.startdate, 'YYYY-MM-') || '01')::DATE, 
        (to_char(dates.enddate + INTERVAL '1 month', 'YYYY-MM-') || '01')::DATE - 1, INTERVAL '1 month') m
) days
-- get months as start date and end date
CROSS JOIN LATERAL (
    SELECT
        days.m::DATE AS startdate,
        (days.m + INTERVAL '1 month')::DATE - 1 AS enddate
) months

如何在两个日期之间的postgresql中获取月，年和日

2 个答案: