sql - GROUP BY 由空格分隔的连续日期

Question

假设您(在 Postgres 9.1 中)有一个这样的表:

date | value 

其中有一些空白(我的意思是:并非 min(date) 和 max(date) 之间的每个可能日期都有它的行)。

我的问题是如何聚合这些数据，以便每个一致的组(没有间隙)被单独处理，如下所示:

min_date | max_date | [some aggregate of "value" column] 

有什么办法吗？我相信使用窗口函数是可能的，但在尝试使用 lag() 和 lead() 一段时间后，我有点卡住了。

例如如果数据是这样的:

 date          | value  
---------------+-------  
 2011-10-31    | 2  
 2011-11-01    | 8  
 2011-11-02    | 10  
 2012-09-13    | 1  
 2012-09-14    | 4  
 2012-09-15    | 5  
 2012-09-16    | 20  
 2012-10-30    | 10  

输出(对于 sum 作为聚合)将是:

   min     |    max     |  sum  
-----------+------------+-------  
2011-10-31 | 2011-11-02 |  20  
2012-09-13 | 2012-09-16 |  30  
2012-10-30 | 2012-10-30 |  10  

Answer 1

create table t ("date" date, "value" int);
insert into t ("date", "value") values
    ('2011-10-31', 2),
    ('2011-11-01', 8),
    ('2011-11-02', 10),
    ('2012-09-13', 1),
    ('2012-09-14', 4),
    ('2012-09-15', 5),
    ('2012-09-16', 20),
    ('2012-10-30', 10);

更简单、更便宜的版本:

select min("date"), max("date"), sum(value)
from (
    select
        "date", value,
        "date" - (dense_rank() over(order by "date"))::int g
    from t
) s
group by s.g
order by 1

我的第一次尝试更加复杂和昂贵:

create temporary sequence s;
select min("date"), max("date"), sum(value)
from (
    select 
        "date", value, d,
        case 
            when lag("date", 1, null) over(order by s.d) is null and "date" is not null 
                then nextval('s')
            when lag("date", 1, null) over(order by s.d) is not null and "date" is not null 
                then lastval()
            else 0 
        end g
    from 
        t
        right join
        generate_series(
            (select min("date") from t)::date, 
            (select max("date") from t)::date + 1, 
            '1 day'
        ) s(d) on s.d::date = t."date"
) q
where g != 0
group by g
order by 1
;
drop sequence s;

输出:

    min     |    max     | sum 
------------+------------+-----
 2011-10-31 | 2011-11-02 |  20
 2012-09-13 | 2012-09-16 |  30
 2012-10-30 | 2012-10-30 |  10
(3 rows)