30

我的 Postgresql 9.1 数据库中有下表：

select * from ro;
date       |  shop_id | amount 
-----------+----------+--------
2013-02-07 |     1001 |      3
2013-01-31 |     1001 |      2
2013-01-24 |     1001 |      1
2013-01-17 |     1001 |      5
2013-02-10 |     1001 |     10
2013-02-03 |     1001 |      4
2012-12-27 |     1001 |      6
2012-12-20 |     1001 |      8
2012-12-13 |     1001 |      4
2012-12-06 |     1001 |      3
2012-10-29 |     1001 |      3

我试图获得一个移动平均线，比较过去 3 个星期四的数据，不包括当前的星期四。这是我的查询：

select date, shop_id, amount, extract(dow from date),
avg(amount) OVER (PARTITION BY extract(dow from date) ORDER BY date DESC
                      ROWS BETWEEN 0 PRECEDING AND 2 FOLLOWING)                          
from ro
where extract(dow from date) = 4

这是给出的结果

date       |  shop_id | amount | date_part |        avg         
-----------+----------+--------+-----------+--------------------
2013-02-07 |     1001 |      3 |         4 | 2.0000000000000000
2013-01-31 |     1001 |      2 |         4 | 2.6666666666666667
2013-01-24 |     1001 |      1 |         4 | 4.0000000000000000
2013-01-17 |     1001 |      5 |         4 | 6.3333333333333333
2012-12-27 |     1001 |      6 |         4 | 6.0000000000000000
2012-12-20 |     1001 |      8 |         4 | 5.0000000000000000
2012-12-13 |     1001 |      4 |         4 | 3.5000000000000000
2012-12-06 |     1001 |      3 |         4 | 3.0000000000000000

我预计

date       |  shop_id | amount | date_part |        avg         
-----------+----------+--------+-----------+--------------------
2013-02-07 |     1001 |      3 |         4 | 2.6666666666666667
2013-01-31 |     1001 |      2 |         4 | 4.0000000000000000
2013-01-24 |     1001 |      1 |         4 | 6.3333333333333333
2013-01-17 |     1001 |      5 |         4 | 6.0000000000000000
2012-12-27 |     1001 |      6 |         4 | 5.0000000000000000
2012-12-20 |     1001 |      8 |         4 |
2012-12-13 |     1001 |      4 |         4 |
2012-12-06 |     1001 |      3 |         4 |
4

2 回答 2

19

SQL小提琴

select
    "date",
    shop_id,
    amount,
    extract(dow from date),
    case when
        row_number() over (order by date) > 3
        then
            avg(amount) OVER (
                ORDER BY date DESC
                ROWS BETWEEN 1 following AND 3 FOLLOWING
            )
        else null end
from (
    select *
    from ro
    where extract(dow from date) = 4
) s

OP的查询有什么问题是框架规范：

ROWS BETWEEN 0 PRECEDING AND 2 FOLLOWING

除此之外，我的查询通过在应用昂贵的窗口函数之前过滤星期四来避免不必要的计算。

如果有必要按 shop_id 进行分区，那么显然在partition by shop_id这两个函数中添加 ，avg并且row_number.

于 2013-02-07T12:02:08.763 回答
12

我认为更好的答案可能是：

SELECT date, shop_id, amount, 
    extract(dow from date) AS dow,
    CASE WHEN count(amount) OVER w = 3 
        THEN avg(amount) OVER w END AS average_amt             
FROM ro
WHERE extract(dow from date) = 4 
WINDOW w AS (ORDER BY date DESC ROWS BETWEEN 1 FOLLOWING AND 3 FOLLOWING)

我认为使用同一个窗口来检查窗口中的行数并取平均值会更干净。（这也节省了两个窗口聚合，如原始答案所示。）

关于在较早的答案中声称“我的查询通过在应用昂贵的窗口函数之前过滤星期四来避免不需要的计算”，这也适用于 OP 建议的查询和我的查询，如附加EXPLAIN到任一显示。

于 2016-07-17T11:32:21.257 回答