如何从 LINQ 实现 SQL CASE 语句

Question

我是 LINQ 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，我正在使用 FromSql 如下调用存储过程。我想知道不使用存储过程是否可以做到这一点？

var 结果 = context.MyData.FromSql("data.GetMyData @pType = {0}, @pLocation = {1}, @pNoOfDays = {2}, @pStartDate = {3}, @pEndDate = {4} ", 类型, 位置, noOfDays, startDate, endDate).ToList();

谢谢。

Answer 1

你可以用这个。

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 2

如果它适合你，你可以使用它。 我只想解释一下 LINQ 查询部分。 您可以将其与 EF 一起使用。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：分组结果的 LINQ 查询。。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 3

下面是对我有用的最终 LINQ 语句。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 });