Question

我是LINQ的新手，我想知道我是否可以从LINQ实现以下SQL查询？

我正在使用Entity Framework Core。

SELECT 0 [All], [Range] =
    CASE  
        WHEN Value BETWEEN 0 AND 25 THEN 'Low' 
        WHEN Value BETWEEN 25 AND 75 THEN 'Medium' 
        WHEN Value BETWEEN 75 AND 90 THEN 'High' 
        WHEN Value BETWEEN 90 AND 100 THEN 'Very High' 
    END 
FROM Result.Calculation C 
    INNER JOIN Data.SampleSet S ON C.SampleSetID = S.ID  
WHERE  S.SampleDrawn >= DATEADD(MONTH,-3,GETDATE()) AND S.Department = 'LOCATION A'

目前，我使用如下的FromSql来调用存储过程。我想知道在不使用存储过程的情况下是否可以这样做？

var result = context.MyData.FromSql（“data.GetMyData @pType = {0}，@ pLocation = {1}，@ pNoOfDays = {2}，@ pStartDate = {3}，@ pndndDate = {4} “，type，location，noOfDays，startDate，endDate）.ToList（）;

感谢。

Answer 1

如果它适合你，你可以使用它。我只想解释LINQ查询部分。你可以在EF上使用它。我为这些创建了虚拟数据。对于EF，请改用IQueryable。

// from a row in first table
// join a row in second table
// on a.Criteria equal to b.Criteria
// where additional conditions
// select the records into these two fields called All and Range
// Convert the result set to list.
var query = (from a in lstCalc
join b in lstSampleSet
on a.SampleSetID equals b.ID where b.SampleDrawn >= DateTime.Now.AddMonths(-8)
&& b.Department == "Location A"
select new { All = 0, Range = Utilities.RangeProvider(a.Value) }).ToList();

编辑：LINQ查询分组结果。。确保您使用的是IQueryable。

 var query = (from a in lstCalc
  join b in lstSampleSet
  on a.SampleSetID equals b.ID where b.SampleDrawn >= DateTime.Now.AddMonths(-8) 
   && b.Department == "Location A"
    group a by Utilities.RangeProvider(a.Value) into groupedData
     select new Result { All = groupedData.Sum(y => y.Value), Range = 
   groupedData.Key }).ToList();

这是相同的代码。

 public class Program
    {
        public static void Main(string[] args) {
            List<Calculation> lstCalc = new List<Calculation>();
            lstCalc.Add(new Calculation() {SampleSetID=1, Value=10 });
            lstCalc.Add(new Calculation() { SampleSetID = 1, Value = 10 });
            lstCalc.Add(new Calculation() { SampleSetID = 2, Value = 20 });
            lstCalc.Add(new Calculation() { SampleSetID = 3, Value = 30 });
            lstCalc.Add(new Calculation() { SampleSetID = 4, Value = 40 });
            lstCalc.Add(new Calculation() { SampleSetID = 5, Value = 50 });
            lstCalc.Add(new Calculation() { SampleSetID = 6, Value = 60 });
            lstCalc.Add(new Calculation() { SampleSetID = 7, Value = 70 });
            lstCalc.Add(new Calculation() { SampleSetID = 8, Value = 80 });
            lstCalc.Add(new Calculation() { SampleSetID = 9, Value = 90 });

            List<SampleSet> lstSampleSet = new List<SampleSet>();
            lstSampleSet.Add(new SampleSet() {Department = "Location A", ID=1, SampleDrawn=DateTime.Now.AddMonths(-5)});
            lstSampleSet.Add(new SampleSet() { Department = "Location A", ID = 2, SampleDrawn = DateTime.Now.AddMonths(-4) });
            lstSampleSet.Add(new SampleSet() { Department = "Location A", ID = 3, SampleDrawn = DateTime.Now.AddMonths(-3) });
            lstSampleSet.Add(new SampleSet() { Department = "Location A", ID = 4, SampleDrawn = DateTime.Now.AddMonths(-2) });
            lstSampleSet.Add(new SampleSet() { Department = "Location A", ID = 5, SampleDrawn = DateTime.Now.AddMonths(-2) });
            lstSampleSet.Add(new SampleSet() { Department = "Location A", ID = 6, SampleDrawn = DateTime.Now.AddMonths(-2) });
            lstSampleSet.Add(new SampleSet() { Department = "Location A", ID = 7, SampleDrawn = DateTime.Now.AddMonths(-1) });

            var query = (from a in lstCalc
                        join b in lstSampleSet
                        on a.SampleSetID equals b.ID where b.SampleDrawn >= DateTime.Now.AddMonths(-8)
                         && b.Department == "Location A"
                        select new { All = 0, Range = Utilities.RangeProvider(a.Value) }).ToList();

            Console.WriteLine(query.Count);
            Console.ReadLine();

        }


    }

        public class Utilities
        {
            public static string RangeProvider(int value)
            {
                if (value > 0 && value <= 25)
                { return "Low"; }
                if (value > 25 && value <= 75)
                { return "Medium"; }
                if (value > 75 && value <= 90)
                { return "High"; }
                else
                { return "Very High"; }
            }

        }

    public class Result {
      public int All { get; set; }
      public string Range { get; set; }
   }

    public class Calculation
    {
        public int SampleSetID { get; set; }
        public int Value { get; set; }

    }

    public class SampleSet
    {
        public int ID { get; set; }
        public DateTime SampleDrawn { get; set; }

        public string Department { get; set; }

    }

Answer 2

你可以使用它。

from C in Calculations
join S in SampleSets on C.SampleSetID equals S.ID 
where S.SampleDrawn >= DateTime.Now.AddMonths(-3)
      && S.Department == "LOCATION A"
select new {
    All = 1 
    , Range = 
        (C.Value >= 0 && C.Value < 25) ? "Low" :
        (C.Value >= 25 && C.Value < 75) ? "Medium" :
        (C.Value >= 75 && C.Value < 90) ? "High" :
        (C.Value >= 90 && C.Value <= 100) ? "Very High" : null
}

Answer 3

以下是对我有用的最终LINQ声明。正如Amit在他的回答中解释的那样 RangeProvider 方法将用于替换SQL CASE语句。

var test2 = (from a in context.Calculations
                         join b in context.SampleSets on a.SampleSetID equals b.ID
                         where b.SampleDrawn >= DateTime.Now.AddDays(-10) && b.Department == "Location A"
                         group a by RangeProvider(a.Value) into groupedData
                         select new { All = groupedData.Count(), Range = groupedData.Key });

如何从LINQ实现SQL CASE语句

3 个答案: