LINQ: Selecting items from a list (Group By/Select/Sum & Max!)

问题

Just getting my head around Linq and having lots of fun! Can any one aid me with a query for this:
I have a list of data:

    Key  Value
    Aaa  12
    AaA  10
    AAa  5
    BBB  2
    Bbb  1

1. I want to group by Key.ToUpper()
2. For every group I need the Max(Value) & Sum(Value)
3. For every group I want to select the entries There the Value != Max(value)
the final result should be like this:

    Key Max Total
    AaA 12  27
    AAa 12  27
    Bbb 2   3

Thanks!

Update, actually I also need the Key from the Maximum entry:

    Key Max Total Correct
    AaA 12  27    Aaa
    AAa 12  27    Aaa
    Bbb 2   3     BBB

回答1:

var results =
  from kvp in source
  group kvp by kvp.Key.ToUpper() into g
  select new
  {
    Group = g,
    Max = g.Max(kvp => kvp.Value),
    Total = g.Sum(kvp => kvp.Value)
  } into ag
  from x in ag.Group  //SelectMany
  where x.Value != ag.Max
    //for the update to the question - note: possibly ambiguous
  let correct = ag.Group.Where(y => y.Value == ag.Max).First().Key
  select new
  {
    Key = x.Key,
    Max = ag.Max,
    Total = ag.Total,
    Correct = correct 
  };

I kinda like the question because of all the little parts (some are rarely used) that are required to make the answer.

Max = g.Max(kvp => kvp.Value),
Total = g.Sum(kvp => kvp.Value)

Performing multiple aggregations on a group is straightforward, yet challenging if you don't know how.

select a into b

This clause takes everything that happened before and starts a new query with the target. Without it, I'd have to start a new query like this:

var A = ... select a

var B = from b in A

It's important to note that the select into clause removes kvp and g from scope.

  from b in source
  from a in b.A  //SelectMany

This "unpacking" of the child collection turns my query about b's into a query about a's. Unlike the default Enumerable.SelectMany overload, it leaves the parent (b) in scope.

where x.Value != ag.Max

Comparing a child's property with a parent's property? Delightful. It's important to remember to break out where anytime you want to filter, even if you just grouped (there is no HAVING).

来源：https://stackoverflow.com/questions/2970848/linq-selecting-items-from-a-list-group-by-select-sum-max

标签

linq

select

group-by