Question

With the following table:

+-------------+-----------+---------+
|  Parent_ID  |  Item_ID  |  Count  |
+-------------+-----------+---------+
|      1      |     1     |    1    |
|      1      |     1     |    5    |
|      1      |     1     |    4    |
|      1      |     2     |    7    |
|      1      |     2     |    2    |
|      2      |     1     |    2    |
|      2      |     1     |    3    |
|      2      |     2     |    2    |
|      2      |     2     |    4    |
+-------------+-----------+---------+

I would like to get the highest available count for given Parent_ID and Item_ID, like the following:

+-------------+-----------+---------+
|  Parent_ID  |  Item_ID  |  Count  |
+-------------+-----------+---------+
|      1      |     1     |    5    |
|      1      |     2     |    7    |
|      2      |     1     |    3    |
|      2      |     2     |    4    |
+-------------+-----------+---------+

How would I go about doing this using LINQ/SQL in C#? For example, I would like the highest item counts for parent id 1. This would be something like:

int parentId = 1;
var counts = from c in database.Items where parentId == c.parentId 
//this gets all counts from items for parent id 1, but I am just looking
//for the highest count per item for parent id 1

Answer 1

To group by multiple fields, you can group using an anonymous object:

var counts = from item in database.Items
             group item by new { item.ParentId, item.ItemId } into itemGroup
             select new Item()
                    {
                        ParentId = itemGroup.Key.ParentId,
                        ItemId = itemGroup.Key.ItemId,
                        Count = itemGroup.Max(x => x.Count)
                    };