FIRST_VALUE 工作正常，但 LAST_VALUE 没有给出想要的结果答案

【问题标题】：FIRST_VALUE working correctly but LAST_VALUE not giving desired resultsFIRST_VALUE 工作正常，但 LAST_VALUE 没有给出想要的结果
【发布时间】：2021-09-08 19:15:55
【问题描述】：

在以下查询中：

SELECT 
    DISTINCT *,
    ROW_NUMBER() OVER (PARTITION BY dog_guid ORDER BY created_at ASC) AS test_number,
    LAG(created_at) OVER (PARTITION BY dog_guid ORDER BY created_at ASC) AS previous_test_date,
    CAST(DATEDIFF(SECOND, LAG(created_at) OVER (PARTITION BY dog_guid ORDER BY created_at ASC), created_at)/(60*60*24) AS FLOAT) AS date_diff_days,
    CAST(DATEDIFF(SECOND, LAG(created_at) OVER (PARTITION BY dog_guid ORDER BY created_at ASC), created_at)/(60) AS FLOAT) AS date_diff_mins,
    FIRST_VALUE(created_at) OVER (PARTITION BY dog_guid ORDER BY created_at ASC) AS first_test_date,
    LAST_VALUE(created_at) OVER (PARTITION BY dog_guid ORDER BY created_at ASC) AS last_test_date
FROM    
    complete_tests c
WHERE
    dog_guid IS NOT NULL

FIRST_VALUE() 函数工作正常，但 LAST_VALUE() 只是给出该列的日期。这是为什么呢？

【问题讨论】：

因为您忘记了LAST_VALUE 中的范围。它的工作方式与文档中的完全一致（并且是正确的）。请参阅解释此确切行为的文档中的 example。
因为默认范围是BETWEEN RANGE UNBOUNDED PRECEDING AND CURRENT ROW，它适用于第一个值但不适用于最后一个值。您可以显式覆盖此范围，但更简单的方法是在相同的订单上执行 FIRST_VALUE()，但 DESC。

标签： sql-server window-functions

【解决方案1】：

如@JeroenMostert 所述，当您将ORDER BY 添加到OVER 子句（采用ROWS/RANGE 的分析函数）时，默认窗口为RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW，因此窗口中的最后一个值始终是当前行。

你想要的是ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING，所以你需要明确地添加它。不直观，但就是这样。

出于性能原因，您还应该将此添加到FIRST_VALUE。

SELECT 
    DISTINCT *,
    ROW_NUMBER() OVER (PARTITION BY dog_guid ORDER BY created_at ASC) AS test_number,
    LAG(created_at) OVER (PARTITION BY dog_guid ORDER BY created_at ASC) AS previous_test_date,
    CAST(DATEDIFF(SECOND, LAG(created_at) OVER (PARTITION BY dog_guid ORDER BY created_at ASC), created_at)/(60*60*24) AS FLOAT) AS date_diff_days,
    CAST(DATEDIFF(SECOND, LAG(created_at) OVER (PARTITION BY dog_guid ORDER BY created_at ASC), created_at)/(60) AS FLOAT) AS date_diff_mins,
    FIRST_VALUE(created_at) OVER (PARTITION BY dog_guid ORDER BY created_at ASC ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) AS first_test_date,
    LAST_VALUE(created_at) OVER (PARTITION BY dog_guid ORDER BY created_at ASC ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) AS last_test_date
FROM    
    complete_tests c
WHERE
    dog_guid IS NOT NULL

严格来说，您可以将其更改为FIRST_VALUE 和DESC，但这会影响性能，首先因为它需要第二次排序，其次因为窗口仍然是RANGE，这需要磁盘工作台。

【讨论】：