通过数据库中的键查询最大值

Question

附下表：

+-------------+-----------+---------+
|  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    |
+-------------+-----------+---------+

我想获得给定 Parent_ID 和 Item_ID 的最高可用计数，如下所示：

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

我将如何在 C# 中使用 LINQ/SQL 执行此操作？例如，我希望父 id 为 1 的项目计数最高。这将类似于：

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

SELECT Parent_ID, Item_ID, MAX(Count)
FROM ...
GROUP BY Parent_Id, Item_ID

Answer 2

select parent_id, 
       item_id, 
       max(count) as count
from your_table
group by parent_id, item_id

Answer 3

要按多个字段分组，您可以使用匿名对象进行分组：

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)
                    };

Answer 4

使用 LINQ2SQL 我想你想要这样的东西：

var counts = database.Items.GroupBy(p => p.parentId).Max(p => p.Key);