我正在使用LINQ to Entities来查询以下MS-SQL表:
RateID RateTypeID Amount EffectiveDate IsRetired
------ ---------- ------ ------------- ---------
1 1 0.565 1/1/2013 0:00 FALSE
2 2 6.000 1/1/2013 0:00 FALSE
3 3 9.000 1/1/2013 0:00 FALSE
4 4 12.000 1/1/2013 0:00 FALSE
5 5 1.500 1/1/2013 0:00 FALSE
6 1 0.550 7/1/2012 0:00 FALSE
7 1 0.495 1/1/2012 0:00 FALSE
8 3 8.000 1/1/2011 0:00 FALSE
9 3 11.000 1/1/2015 0:00 FALSE
10 5 2.000 7/1/2013 0:00 TRUE
我想构建一个函数,它将使用LINQ返回IQueryable(Of Rate)
,我可以将其加入其他查询,以便RateTypeID
有效地确定应用于每个项目的当前速率。 IsRetired
必须为false,我需要最近的EffectiveDate
行。
这是我得到的最接近的(LINQPad VB语句):
Dim dateNow As Date = Date.Now
Dim result = (
From r In Rates
Where r.IsRetired = False And r.EffectiveDate <= dateNow
Group r By r.RateTypeID
Into rg = Group
Select New With {
.cr = rg.OrderByDescending(Function(r) r.EffectiveDate).Take(1)
}
)
result.Dump()
此查询返回DbQuery(Of AnonymousType_1(Of IEnumerable(Of Rate)))
(每个包含1个费率项的集合集合),这不是我想要的。
这个Q&amp; A解决了一个非常类似的问题但是在SQL而不是LINQ中: How can I SELECT rows with MAX(Column value), DISTINCT by another column in SQL?
我在vb.net编码,但也会接受c#答案。谢谢!
答案 0 :(得分:6)
使用Take()
返回IEnumerable<T>
,FirstOrDefault()
只返回<T>
,所以:
IQueryable<RecentRate> query = from r in Rates
where r.IsRetired == false && r.EffectiveDate <= dateNow
group r by r.RateTypeID into rg
select new RecentRate
{
Rate = rg.OrderByDescending(r => r.EffectiveDate).FirstOrDefault()
};
这可能有效:
IQueryable<Rate> query = from r in Rates
where r.IsRetired == false && r.EffectiveDate <= dateNow
group r by r.RateTypeID into rg
select rg.OrderByDescending(r => r.EffectiveDate).FirstOrDefault();
我认为这是VB语法
Dim result = (
From r In Rates
Where r.IsRetired = False And r.EffectiveDate <= dateNow
Group r By r.RateTypeID
Into rg = Group
Select rg.OrderByDescending(Function(r) r.EffectiveDate).FirstOrDefault()
)