如何将LINQ左外连接限制为一行

Question

我有一个左外连接(下面)按预期返回结果.我需要将"右"表的结果限制为"第一次"命中.我能以某种方式这样做吗？目前,我得到两个表中每个记录的结果,我只想看左边表中的一个结果(项目),无论我在右表(照片)中有多少结果.

        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();
}

当我针对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

当我询问该计划时,它显示子查询是通过此连接实现的:

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

Answer 2

您要对表进行分组。最好的方法是：

    var query = from i in db.items
                join p in (from p in db.photos
                           group p by p.item_id into gp
                           where gp.Count() > 0
                           select new { item_id = g.Key, Photo = g.First() })
            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.Photo.PhotoID.ToString()
            };

编辑：这是艾米B说话。我之所以这样做，是因为尼克要求我这样做。尼克，请根据需要修改或删除此部分。

生成的SQL很大。通过参数传入int 0（与计数进行比较）。

SELECT [t0].X AS [id], CONVERT(NVarChar(MAX),(
    SELECT [t6].Y
    FROM (
        SELECT TOP (1) [t5].Y
        FROM [dbo].[Photos] AS [t5]
        WHERE (([t4].Y IS NULL) AND ([t5].Y IS NULL)) OR (([t4].Y IS NOT NULL) AND ([t5].Y IS NOT NULL) AND ([t4].Y = [t5].Y))
        ) AS [t6]
    )) AS [PhotoId]
FROM [dbo].[Items] AS [t0]
CROSS APPLY ((
        SELECT NULL AS [EMPTY]
        ) AS [t1]
    OUTER APPLY (
        SELECT [t3].Y
        FROM (
            SELECT COUNT(*) AS [value], [t2].Y
            FROM [dbo].[Photos] AS [t2]
            GROUP BY [t2].Y
            ) AS [t3]
        WHERE (([t0].X) = [t3].Y) AND ([t3].[value] > @p0)
        ) AS [t4])
ORDER BY [t0].Z DESC

执行计划揭示了三个左连接。至少一个是微不足道的，不应计入（它为零）。这里有足够的复杂性，我无法明确指出效率方面的任何问题。它可能运行得很好。