假设我们有两个List<T>。第一个是销售总额列表：
class SalesTotals
{
public Guid EmpID { get; set; }
public string EmpName { get; set; }
public decimal? TotalSales { get; set; }
}
然后我们按年度列出另一份销售清单：
class YearlySales
{
public Guid EmpID { get; set; }
public short SalesYear { get; set; }
public decimal? YearlyTotals { get; set; }
}
这些用于一起创建一个＆＃34;交叉标签＆＃34;报告列出每位员工的总销售额，然后列出每年的相关年度销售额列。它看起来像这样：
| Name | Total | 2018 | 2017 | 2016 |
+------+-------+------+------+------+
| Joe | 70 | 20 | | 50 |
| Sam | 60 | 30 | 20 | 10 |
| Fred | 50 | 30 | | 20 |
| Bob | 40 | 10 | 15 | 15 |
默认情况下，报表按TotalSales排序（没问题）。但如果我们想按个别年份排序，事情会变得棘手。按2017年排序（然后按总计）：
| Name | Total | 2018 | 2017 | 2016 |
+------+-------+------+------+------+
| Sam | 60 | 30 | 20 | 10 |
| Bob | 40 | 10 | 15 | 15 |
| Joe | 70 | 20 | | 50 |
| Fred | 50 | 30 | | 20 |
我假设我们希望（左）Join这两个List<T>位于EmpID，where SalesYear == <year to sort by>然后OrderBy YearlyTotals, TotalSales（因为YearlyTotals）可能在某一年内不存在，在这种情况下我们仍然需要某种类型的订单）。因此，我们还必须考虑当年可能没有记录加入（因此它需要是一个左连接）。
如果我正在编写SQL，它看起来像这样：
SELECT ST.EmpID, ST.EmpName, ST.TotalSales
FROM SalesTotals AS ST
LEFT JOIN YearlySales AS YS ON ST.EmpID=YS.EmpID
WHERE YS.SalesYear=@SortBySalesYear OR YS.SalesYear IS NULL
ORDER BY YS.YearlySales DESC, ST.TotalSales DESC
我对Linq（还）不够好，能够解决这个问题。事实上，我几乎没有得到任何地方（可能一次尝试做太多，也许我需要将其分解为单个步骤，而不是搜索一个班轮）。
那么，有没有办法用Linq做到这一点？或者我应该尝试其他类型的方法吗？
注意：我需要的只是一个＆＃34;到位＆＃34;在这里排序。我不需要/想要在此处返回不同类型的List<T>，只是排序List<SalesTotals>。
编辑：我更喜欢Linq＆＃34;查询语法＆＃34;因为它对我来说更直观（强大的SQL背景）。所以我更喜欢使用查询语法而不是方法语法的答案。
编辑：这是一个测试用例设置：
class SalesTotals
{
public int EmpID { get; set; }
public string EmpName { get; set; }
public decimal? TotalSales { get; set; }
}
class YearlySales
{
public int EmpID { get; set; }
public short SalesYear { get; set; }
public decimal? YearlyTotals { get; set; }
}
class TestSort
{
public TestSort()
{
var st = new List<SalesTotals>
{
new SalesTotals() { EmpID = 1, EmpName = "Joe", TotalSales = 70 },
new SalesTotals() { EmpID = 2, EmpName = "Sam", TotalSales = 60 },
new SalesTotals() { EmpID = 3, EmpName = "Fred", TotalSales = 50 },
new SalesTotals() { EmpID = 4, EmpName = "Bob", TotalSales = 40 }
};
var ys = new List<YearlySales>
{
new YearlySales() { EmpID = 1, SalesYear = 2018, YearlyTotals = 20 },
new YearlySales() { EmpID = 2, SalesYear = 2018, YearlyTotals = 30 },
new YearlySales() { EmpID = 3, SalesYear = 2018, YearlyTotals = 30 },
new YearlySales() { EmpID = 4, SalesYear = 2018, YearlyTotals = 10 },
new YearlySales() { EmpID = 2, SalesYear = 2017, YearlyTotals = 20 },
new YearlySales() { EmpID = 4, SalesYear = 2017, YearlyTotals = 15 },
new YearlySales() { EmpID = 1, SalesYear = 2016, YearlyTotals = 10 },
new YearlySales() { EmpID = 2, SalesYear = 2016, YearlyTotals = 15 },
new YearlySales() { EmpID = 3, SalesYear = 2016, YearlyTotals = 50 },
new YearlySales() { EmpID = 4, SalesYear = 2016, YearlyTotals = 20 }
};
st = SortByYear(st, ys, 2017);
}
private List<SalesTotals> SortByYear(List<SalesTotals> salesTotals, List<YearlySales> yearlySales, short sortYear)
{
// return sorted salesTotals by sortYear using both salesTotals and yearlySales joined on EmpID
}
}
答案 0 :(得分：2)
不是直接转换SQL，我认为将查询分为两部分会更清楚。
首先，找到要排序的年份的YearlySales：
var sortYearSales = from ys in yearlySales
where ys.SalesYear == SortBySalesYear
select ys;
然后你可以保持联接并排序（因为ys可能是null，我使用了空条件成员acecss运算符）：
var orderedSalesTotals = (from st in salesTotals
join ys in sortYearSales on st.EmpID equals ys.EmpID into ysj
from ys in ysj.DefaultIfEmpty()
orderby ys?.YearSales descending, st.TotalSales descending
select st).ToList();
注意：我将YearlySales成员的名称更改为YearSales，因为C＃编译器抱怨该成员和类具有相同的名称。
您可以在单个查询中执行此操作，但您必须将第一个查询嵌套到第二个查询中，或者在查询中使用lambda语法：
var orderedSalesTotals = (from st in salesTotals
join ys in yearlySales on st.EmpID equals ys.EmpID into ysj
from ys in ysj.Where(y => y.SalesYear == SortBySalesYear).DefaultIfEmpty()
orderby ys?.YearSales descending, st.TotalSales descending
select st).ToList();
答案 1 :(得分：1)
你可以像在SQL中一样编写它！
var results = from t in totals
join y in years on t.EmpID equals y.EmpID into groupedTable
from p in groupedTable.DefaultIfEmpty()
where y == null || y.SalesYear == year
orderby y.SalesYear, t.TotalSales descending
select t;
快速注意：默认情况下，LINQ中的联接是内部联接。如果需要外连接，则必须使用DefaultIfEmpty（）调用。
答案 2 :(得分：-1)
有点作品。需要为销售提供空值
List<YearlySale> YearlySales = new List<YearlySale>() { new YearlySale() { EmpID = 1, Sales = 700, Year = 2018 },
new YearlySale() { EmpID = 1, Sales = 600, Year = 2017 },
new YearlySale() { EmpID = 1, Sales = 500, Year = 2016 },
new YearlySale() { EmpID = 2, Sales = 400, Year = 2018 },
new YearlySale() { EmpID = 2, Sales = null, Year = 2017 },
new YearlySale() { EmpID = 2, Sales = 300, Year = 2016 }
};
List<SalesTotal> SalesTotals = new List<SalesTotal>() { new SalesTotal() { EmpID = 1, EmpName = "stan", TotalSales = 1800 },
new SalesTotal() { EmpID = 2, EmpName = "sally", TotalSales = 700 }
};
var q = from s in SalesTotals
join y18 in YearlySales
on s.EmpID equals y18.EmpID
join y17 in YearlySales
on s.EmpID equals y17.EmpID
join y16 in YearlySales
on s.EmpID equals y16.EmpID
where y18.Year == 2018
where y17.Year == 2017
where y16.Year == 2016
select new { SalesTotal = s, Year18 = y18 == null ? 0 : y18.Year, YearS18 = y18 == null ? 0 : y18.Sales
, Year17 = y17 == null ? 0 : y17.Year, YearS17 = y17 == null ? 0 : y17.Sales
, Year16 = y16 == null ? 0 : y16.Year, YearS16 = y16 == null ? 0 : y16.Sales
};
foreach (var v in q.OrderBy(x => x.SalesTotal.EmpID))
{
Debug.WriteLine($"{v.SalesTotal.EmpID} {v.SalesTotal.EmpName} {v.SalesTotal.TotalSales} {v.YearS18} as y18 {v.YearS17} as y17 {v.YearS16} as y16" );
}