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“正确”表中的结果限制为“第一次”命中。我能以某种方式做到这一点吗？目前，我为两个表中的每条记录都得到了一个结果，无论我在右表（照片）中有多少结果，我都只想从左边的表（项目）中看到一个结果。

        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();
}

当我针对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 name 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">

Answer 2

您要做的是对表格进行分组。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 Amy B 说话。我这样做只是因为尼克要求我这样做。 Nick，请在您认为合适的情况下修改或删除此部分。

生成的 SQL 很大。 int 0（与计数比较）通过参数传入。

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

执行计划显示三个左连接。至少一个是微不足道的，不应该被计算（它带来了零）。这里有足够的复杂性，我无法清楚地指出任何效率问题。它可能会运行得很好。

Answer 3

你可以这样做：

var q = from c in
          (from s in args
           select s).First()
        select c;

围绕查询的最后一部分。不确定它是否会起作用或会产生什么样的怪异 SQL :)

Answer 4

使用内部查询。在没有照片的情况下包括DefaultIfEmpty，在多于一张的情况下包括orderby。以下示例拍摄id 最大的照片。

var query = 
    from i in db.items
    let p = from p in db.photos where i.id == p.item_id orderby p.id select p).DefaultIfEmpty().Last()
    orderby i.date descending
    select new {
      itemName = i.name,
      itemID = i.id,
      id = i.id,
      photoID = p.PhotoID
    };

如果需要专门处理无照片的情况，可以省略DefaultIfEmpty，改用FirstOrDefault/LastOrDefault。