Question

我有一种情况，我需要获取FromCountry和ToCountry的组合的最大日期记录的FXRate。

    FromCountry    ToCountry      FXRate         FXDate
   --------------------------------------------------------
    A               B             1.14           1/1/2019
    A               B             1.13           2/1/2019
    A               B             1.12           3/1/2019
    A               B             1.11           4/1/2019 -- Expected Record for 'A' and 'B'
    c               D             1.16           1/1/2019
    c               D             1.17           2/1/2019
    c               D             1.18           3/1/2019
    c               D             1.19           4/1/2019 -- Expected Record for 'C' and 'D'

问题是“ FXView”视图有很多记录，而我目前的方法是这样的

SELECT
       fx.FromCountry
     , fx.ToCountry
     , fx.FXDate
     , fx.FXRate
FROM
(
SELECT
        FromCountry
      , Tocountry
      , Max(FXDate) as MaxFXDate 
FROM FXView
GROUP BY 
        FromCountry
      , Tocountry
)result
INNER JOIN FXView fx
   ON   result.FromCountry = fx.FromCountry
   AND  result.ToCountry   = fx.ToCountry
   AND  result.MaxFXDate   = fx.FXDate

我也尝试使用“最大”窗口功能，然后获得它的“独特性”，但这也需要时间。

对于这种性能，是否有更好的解决方案？

注意：此视图中没有任何自然键。

Answer 1

您可以尝试如下所示的公用表表达式，在某些情况下，它有助于提高性能

WITH result(FromCountry,ToCountry, MaxFXDate) AS(
SELECT
        FromCountry
      , Tocountry
      , Max(FXDate) as MaxFXDate 
FROM FXView
GROUP BY 
        FromCountry
      , Tocountry
)

SELECT
       fx.FromCountry
     , fx.ToCountry
     , fx.FXDate
     , fx.FXRate
FROM
result
INNER JOIN FXView fx
   ON   result.FromCountry = fx.FromCountry
   AND  result.ToCountry   = fx.ToCountry
   AND  result.MaxFXDate   = fx.FXDate

Answer 2

它是否甚至需要分组或加入？

SELECT TOP 1 WITH TIES
   fx.FromCountry
 , fx.ToCountry
 , fx.FXDate
 , fx.FXRate
FROM FXView fx
ORDER BY ROW_NUMBER() OVER (PARTITION BY fx.FromCountry, fx.ToCountry ORDER BY fx.FXDate DESC)

性能调整，获取Max Value的详细信息

2 个答案: