下面是最后一个LINQ语句，它为我工作。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 });

可以使用此自定义表达式生成器方法

public static Expression<Func<TEntity, TResult>> SwitchCase<TEntity, TField, TResult>(string fieldName, List<(TField Value, TResult Result)> conditions, TResult defaultValue)
           where TEntity : EntityBase
        {
            // Create parameter expression for TEntity
            var entityParameter = Expression.Parameter(typeof(TEntity), "x");

            // Create expression to extract field property from TEntity
            var fieldExpr = Expression.Convert(Expression.Property(entityParameter, fieldName), typeof(TField));

            // Define default expression for when no conditions match
            var defaultExpr = Expression.Constant(defaultValue);

            // Build conditional expressions by nesting Expression.Condition

            //example x.Status == 0 ? "New" : (x.Status == 1 ? "Active" : "default")
            Expression conditionExpr = defaultExpr;
            for (int i = conditions.Count - 1; i >= 0; i--)
            {
                var mapping = conditions[i];
                var testExpr = Expression.Equal(fieldExpr, Expression.Constant(mapping.Value));
                conditionExpr = Expression.Condition(testExpr, Expression.Constant(mapping.Result), conditionExpr);
            }

            // Create lambda expression for use in LINQ query
            var lambdaExpr = Expression.Lambda<Func<TEntity, TResult>>(conditionExpr, entityParameter);

            return lambdaExpr;
        }

使用方法：

var mappings = new List<(int Number, string Name)>
{
     (0, "New"),
     (1, "Active"),
     (2, "Closed")
};

var result = await context.Entities.OrderBy(CustomExpressionsBuilder.SwitchCase<Entity, int, string>("Status", mappings, "default")).ToListAsync()

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
}

你可以使用它，如果它适合你。我只会解释LINQ查询部分。你可以使用EF。我为这些创建了虚拟数据。对于EF，使用IQueryable代替。

// 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();

EDIT：LINQ Query for grouped result..请确保您使用的是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();

下面是相同的代码。

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; }

    }