Question

想象一下作者和物品之间的多对多关系。

查询项目的作者时，似乎有两种广泛的方法（下面的例子）。

为什么我可以选择一种方法而不是另一种？

下面两个查询的SQL生成方式有什么不同吗？

接近一个

按ID查找项目，然后使用Authors属性列出作者。

List<Authors> Authors = db.Items
    .Where(i => i.ID == CurrentItemID)
    .FirstOrDefault()
    .Authors.ToList();

接近两个

查找包含当前项目ID的AuthoredItems的所有作者。

List<Authors> Authors = db.Authors
    .Where(i => i.AuthoredItems.Any(a => a.ID == CurrentItemID))
    .ToList();

多对多关系如此：

public class Item
{
    ...
    ICollection<Author> Authors {get;set;}
}

public class Author
{
    ...
    ICollection<Item> AuthoredItems {get;set;}
}

更新

正如所建议的，我试图为这些查询打印生成的sql。

方法1给了我：

SELECT 
[Project1].[ID] AS [ID], [Project1].[Title] AS [Title], 
[Project1].[C1] AS [C1], [Project1].[ID1] AS [ID1], 
[Project1].[Name] AS [Name] 

FROM ( 
  SELECT [Extent1].[ID] AS [ID], 
    [Extent1].[Title] AS [Title], 
    [Join1].[ID] AS [ID1], 
    [Join1].[Name] AS [Name], 
    CASE WHEN ([Join1].[Author_ID] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C1] FROM [dbo].[Items] AS [Extent1] 

  LEFT OUTER JOIN (
    SELECT [Extent2].[Author_ID] AS [Author_ID], 
    [Extent2].[Item_ID] AS [Item_ID], 
    [Extent3].[ID] AS [ID], [Extent3].[Name] AS [Name] 
    ROM [dbo].[AuthoredItems] AS [Extent2] 
    INNER JOIN [dbo].[Authors] AS [Extent3] ON [Extent3].[ID] = [Extent2].[Author_ID] 
  ) 
  AS [Join1] ON [Extent1].[ID] = [Join1].[Item_ID] 
  WHERE 1 = [Extent1].[ID] 
) 
AS [Project1] 
ORDER BY [Project1].[ID] ASC, [Project1].[C1] ASC

方法2给了我：

SELECT [Extent1].[ID] AS [ID], [Extent1].[Name] AS [Name] 
FROM [dbo].[Authors] AS [Extent1] 
WHERE EXISTS (
  SELECT 1 AS [C1] FROM [dbo].[AuthoredItems] AS [Extent2] 
  WHERE ([Extent1].[ID] = [Extent2].[Author_ID]) AND (1 = [Extent2].[Item_ID]) 
)

第二种方法的sql在我的大脑上更容易。加入的次数较少等等。我认为第二种方法更有效吗？

Answer 1

第一个查询似乎更“自然”且更容易理解，因为您知道项目ID，一旦找到该项目，您只需选择所有列出的作者。

在另一个查询中，如果他是该项目的作者，则查看每个作者，如果是，则选择他。

我会选择第一个。我无法确定两个查询中的速度差异，但第一个查询肯定更容易理解。

这2个linq查询之间有什么区别吗？

1 个答案: