c# - 如何将 LINQ 左外连接限制为一行

Question

我有一个左外连接(如下)按预期返回结果。我需要将“右”表的结果限制为“第一”命中。我能以某种方式做到吗？目前，我得到了两个表中每条记录的结果，无论我在右表(照片)中有多少结果，我只想看到左侧表(项目)中的一个结果。

        var query = from i in db.items
                join p in db.photos
                on i.id equals p.item_id into tempPhoto
                from tp in tempPhoto.DefaultIfEmpty()
                orderby i.date descending 
                select new
                {
                    itemName = i.name,
                    itemID = i.id,
                    id = i.id,
                    photoID = tp.PhotoID.ToString()
                };


    GridView1.DataSource = query;
    GridView1.DataBind();

Answer 1

这将为您完成这项工作。

from i in db.items
let p = db.photos.Where(p2 => i.id == p2.item_id).FirstOrDefault()
orderby i.date descending
select new
{
  itemName = i.name,
  itemID = i.id,
  id = i.id,
  photoID = p == null ? null : p.PhotoID.ToString();
}

当我根据我自己的模型生成它时，我得到了这个 sql(并且在投影中没有名称和第二个 id 列)。

SELECT [t0].[Id] AS [Id], CONVERT(NVarChar,(
    SELECT [t2].[PhotoId]
    FROM (
        SELECT TOP (1) [t1].[PhotoId]
        FROM [dbo].[Photos] AS [t1]
        WHERE [t1].[Item_Id] = ([t0].[Id])
        ) AS [t2]
    )) AS [PhotoId]
FROM [dbo].[Items] AS [t0]
ORDER BY [t0].[Id] DESC

查询计划时，显示子查询是通过这个join实现的:

<RelOp LogicalOp="Left Outer Join" PhysicalOp="Nested Loops">