sql - 缺少数据的窗口函数

Question

假设我有一个表(MyTable)如下:

item_id  |  date
----------------
     1   | 2016-06-08
     1   | 2016-06-07
     1   | 2016-06-05
     1   | 2016-06-04
     1   | 2016-05-31
     ...
     2   | 2016-06-08
     2   | 2016-06-06
     2   | 2016-06-04
     2   | 2016-05-31
     ...
     3   | 2016-05-31
     ...

我想建立一个每周汇总表，报告运行 7 天的窗口。该窗口基本上会显示“在过去 7 天内报告了多少个唯一的 item_id”？

因此，在这种情况下，输出表将类似于:

date      | weekly_ids
----------------------
2016-05-31|  3       # All 3 were present on the 31st
2016-06-01|  3       # All 3 were present on the 31st which is < 7 days before the 1st
2016-06-02|  3       # Same
2016-06-03|  3       # Same
2016-06-04|  3       # Same
2016-06-05|  3       # Same
2016-06-06|  3       # Same
2016-06-07|  3       # Same
2016-06-08|  2       # item 3 was not present for the entire last week so it does not add to the count.

我试过:

SELECT
    item_id,
    date,
    MAX(present) OVER (
        PARTITION BY item_id
        ORDER BY date
        ROWS BETWEEN 6 PRECEDING AND CURRENT ROW) AS is_present
FROM (
    # Inner query
    SELECT
        item_id,
        date,
        1 AS present,
    FROM MyTable
)
GROUP BY date
ORDER BY date DESC

这感觉好像在朝着正确的方向前进。但实际上，当日期不存在(日期太多)时，窗口会在错误的时间范围内运行，并且当 item_id 不存在时，它也不会输出日期记录(即使它出现在前一个日期)。这个问题有简单的解决方法吗？

如果有帮助和必要

• 我可以硬编码最早的日期
• 我还可以获得包含所有存在的 item_id 的表格。
• 此查询将仅在 BigQuery 上运行，因此 BQ 特定函数/语法是公平的游戏，不幸的是，不在 BigQuery 上运行的 SQL 函数/语法对我没有帮助......

Answer 1

我已经创建了一个临时表来保存日期，但是，您可能会从为这些连接添加一个永久表到数据库中受益。相信我，它会减少头痛。

DECLARE @my_table TABLE
(
    item_id int,
    date DATETIME
)
INSERT @my_table SELECT 1,'2016-06-08'
INSERT @my_table SELECT 1,'2016-06-07'
INSERT @my_table SELECT 1,'2016-06-05'
INSERT @my_table SELECT 1,'2016-06-04'
INSERT @my_table SELECT 1,'2016-05-31'
INSERT @my_table SELECT 2,'2016-06-08'
INSERT @my_table SELECT 2,'2016-06-06'
INSERT @my_table SELECT 2,'2016-06-04'
INSERT @my_table SELECT 2,'2016-05-31'
INSERT @my_table SELECT 3,'2016-05-31'

DECLARE @TrailingDays INT=7
DECLARE @LowDate DATETIME='01/01/2016'
DECLARE @HighDate DATETIME='12/31/2016'

DECLARE @Calendar TABLE(CalendarDate DATETIME)
DECLARE @LoopDate DATETIME=@LowDate

WHILE(@LoopDate<=@HighDate) BEGIN
    INSERT @Calendar SELECT @LoopDate
    SET @LoopDate=DATEADD(DAY,1,@LoopDate)
END 



SELECT
    date=HighDate,      
    weekly_ids=COUNT(DISTINCT item_id)
FROM
(
    SELECT
        HighDate=C.CalendarDate,
        LowDate=LAG(C.CalendarDate, @TrailingDays,0) OVER (ORDER BY C.CalendarDate)
    FROM
        @Calendar C 
    WHERE
        CalendarDate BETWEEN @LowDate AND @HighDate
)AS X
LEFT OUTER JOIN @my_table MT ON MT.date BETWEEN LowDate  AND HighDate
GROUP BY
    LowDate,
    HighDate