SQL：自动填充日期之间的价格答案

【问题标题】：SQL : automatically fill price between datesSQL：自动填充日期之间的价格
【发布时间】：2020-06-09 07:39:03
【问题描述】：

我正在尝试从两个表中编写一个视图，一个包含产品 ID 和周的参考表：

+------------+------+
| Product_id | week |   t1
+------------+------+
|          1 |    1 |
|          2 |    1 |
|          1 |    2 |
|          2 |    2 |
|          1 |    3 |
|          2 |    3 |
+------------+------+ etc...

另一个包含产品 ID、产品价格变化的周数和价格

+------------+------+-------+
| Product_id | week | price | t2
+------------+------+-------+
|          1 |    1 |    70 |
|          1 |    2 |    50 |
|          2 |    2 |    70 |
|          1 |    4 |    30 |
|          2 |    4 |    40 |
+------------+------+-------+

我知道如何通过加入这两个表来轻松实现这一点：

+------------+------+-------+
| Product_id | week | price | 
+------------+------+-------+
|          1 |    1 |    70 |
|          1 |    2 |    50 |
|          1 |    3 |       |
|          1 |    4 |    30 |
|          1 |    5 |       |
|          2 |    1 |       |
|          2 |    2 |    70 |
|          2 |    3 |       |
|          2 |    4 |    40 |
|          2 |    5 |       |
+------------+------+-------+

但我的目标是填补空白并获得每周的价格（不创建任何新表），例如：

+------------+------+-------+
| Product_id | week | price |
+------------+------+-------+
|          1 |    1 |    70 |
|          1 |    2 |    50 |
|          1 |    3 |    50 |
|          1 |    4 |    30 |
|          1 |    5 |    30 |
|          2 |    1 |       |
|          2 |    2 |    70 |
|          2 |    3 |    70 |
|          2 |    4 |    40 |
|          2 |    5 |    40 |
+------------+------+-------+ (product 2 isn't sold yet at week 1, so it doesn't have a price).

我不知道如何在 SQL 中执行此操作。我还没有使用 PARTITION BY 或 LAG ，这可能是我正在寻找的。如果有人能把我推向正确的方向，我将不胜感激:)

【问题讨论】：

标签： sql select teradata window-functions

【解决方案1】：

您可以使用窗口函数 - teradata 支持的 ignore nulls 子句在这里派上用场：

select
    t1.product_id,
    t1.week,
    coalesce(
        t2.price,
        lag(t2.price ignore nulls) over(partition by t1.product_id order by t1.week)
    ) price
from t1
left join t2 
    on  t2.product_id = t1.product_id
    and t2.week = t1.week

或者更好的是，正如 dnoeth 所建议的，您可以使用last_value()，这样就不需要coalesce()：

select
    t1.product_id,
    t1.week,
    last_value(t2.price ignore nulls) over(partition by t1.product_id order by t1.week) price
from t1
left join t2 
    on  t2.product_id = t1.product_id
    and t2.week = t1.week

【讨论】：

无需 COALESCE/LAG：last_value(t2.price ignore nulls) over(partition by t1.product_id order by t1.week)
@dnoeth：是的，好点，这更简单。答案已更新 - 有学分。谢谢！

【解决方案2】：

使用cross join 生成行，然后使用left join 和窗口函数：

with weeks as (
      select row_number() over (order by product_id) as n
      from table1
     )
select t1.product_id, w.n as week,
       coalesce(t2.price, lag(t2.price ignore nulls) over (partition by p.product_id order by w.n)
               ) as price
from (select distinct product_id
      from table1 t1
     ) p cross join
     weeks w left join
     table2 t2
     on t2.product_id = p.product_id and t2.week = w.week
where w.n <= 5

【讨论】：

【解决方案3】：

您可以使用LEFT JOIN 来完成此操作。

SELECT t1.Product_id, t1.week, tmp.price
FROM t1
LEFT JOIN t2 tmp ON tmp.Product_id = t1.Product_id AND
                    tmp.week = (SELECT MAX(week) FROM t2
                                WHERE Product_id = tmp.Product_id AND week <= t1.week)
ORDER BY t1.Product_id, t1.week

我认为OUTER APPLY 会更干净，但我不知道 teradata 是否支持。

SELECT t1.Product_id, t1.week, oa.price
FROM t1
OUTER APPLY (SELECT TOP 1 price FROM t2
             WHERE Product_id = t1.Product_id AND week <= t1.week
             ORDER BY week DESC) oa
ORDER BY t1.Product_id, t1.week

【讨论】：