Select multiple fields group by and sum

问题

I want to do a query with linq (list of objects) and I really don't know how to do it, I can do the group and the sum but can't select rest of the fields. Example:

ID  Value     Name   Category
1   5         Name1  Category1  
1   7         Name1  Category1
2   1         Name2  Category2
3   6         Name3  Category3
3   2         Name3  Category3

I want to group by ID, SUM by Value and return all fields like this.

ID  Value     Name   Category
1   12        Name1  Category1  
2   1         Name2  Category2
3   8         Name3  Category3

回答1:

Updated : If you're trying to avoid grouping for all the fields, you can group just by Id:

data.GroupBy(d => d.Id)
    .Select(
        g => new
        {
            Key = g.Key,
            Value = g.Sum(s => s.Value),
            Name = g.First().Name,
            Category = g.First().Category 
        });

But this code assumes that for each Id, the same Name and Category apply. If so, you should consider normalizing as @Aron suggests. It would imply keeping Id and Value in one class and moving Name, Category (and whichever other fields would be the same for the same Id) to another class, while also having the Id for reference. The normalization process reduces data redundancy and dependency.

回答2:

void Main()
{
            //Me being lazy in init
    var foos = new []
    {
        new Foo { Id = 1, Value = 5},
        new Foo { Id = 1, Value = 7},
        new Foo { Id = 2, Value = 1},
        new Foo { Id = 3, Value = 6},
        new Foo { Id = 3, Value = 2},
    };
    foreach(var x in foos)
    {
        x.Name = "Name" + x.Id;
        x.Category = "Category" + x.Id;
    }
            //end init.

    var result = from x in foos
                group x.Value by new { x.Id, x.Name, x.Category}
                into g
                select new { g.Key.Id, g.Key.Name, g.Key.Category, Value = g.Sum()};
    Console.WriteLine(result);
}

// Define other methods and classes here
public class Foo
{
    public int Id {get;set;}
    public int Value {get;set;}

    public string Name {get;set;}
    public string Category {get;set;}   
}

回答3:

try this:

var objList = new List<SampleObject>();

objList.Add(new SampleObject() { ID = 1, Value = 5, Name = "Name1", Category = "Catergory1"});
objList.Add(new SampleObject() { ID = 1, Value = 7, Name = "Name1", Category = "Catergory1"});
objList.Add(new SampleObject() { ID = 2, Value = 1, Name = "Name2", Category = "Catergory2"});
objList.Add(new SampleObject() { ID = 3, Value = 6, Name = "Name3", Category = "Catergory3"});
objList.Add(new SampleObject() { ID = 3, Value = 2, Name = "Name3", Category = "Catergory3"});

var newList = from val in objList
              group val by new { val.ID, val.Name, val.Category } into grouped
              select new SampleObject() { ID = grouped.ID, Value = grouped.Sum(), Name = grouped.Name, Category = grouped.Category };

to check with LINQPad:

newList.Dump();

回答4:

If your class is really long and you don't want to copy all the stuff, you can try something like this:

l.GroupBy(x => x.id).
  Select(x => {
    var ret = x.First();
    ret.value = x.Sum(xt => xt.value);
    return ret;
  }).ToList();

With great power great responsibility comes. You need to be careful. Line ret.value = x.Sum(xt => xt.value) will change your original collection, as you are passing reference, not new object. If you want to avoid it, you need to add some Clone method into your class like MemberwiseClone (but again, this will create shallow copy, so be careful). Afer that just replace the line with: var ret = x.First().Clone();

来源：https://stackoverflow.com/questions/16100900/select-multiple-fields-group-by-and-sum

标签

ASP.NET

linq

linq-group