返回给定日期的正确数据。

Question

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我有两个表一个调用Employee，另一个表调用Target Ratio。它们通过FK EmployeeID

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tblEmployee
EmployeeID FirstName LastName
1              John     Doe

tblTargetRatio
TargetRatioID EmployeeID EffectiveDate Ratio
      1            1        1/1/2012     8
      2            1        6/1/2012     5
      3            1        9/1/2012     7

我的问题是如何查询tblTargetRatio表以返回以下情况的正确记录：

1 EmployeeID = 1 and Date = 03/12/2012 (Expecting Ratio = 8)
2 EmployeeID = 1 and Date = 10/10/2012 (Expecting Ratio = 7)
3 EmployeeID = 1 and Date = 08/12/2012 (Expecting Ratio = 5)

谢谢， 艾伦

Answer 1

看来您正在寻找最近的生效日期。您可以通过查找小于搜索值的最大值（其中@Date是搜索日期）来执行此操作：

select top 1 * from tblTargetRatio
where employeeid = 1 and effectivedate < @Date
order by effectivedate desc

演示：http://www.sqlfiddle.com/#!3/f6d4b/2