[英]Is there any difference between these 2 linq queries?
想象一下作者和项目之间的多对多关系。
查询项目的作者时,似乎有2种广泛的方法(以下示例)。
为什么我会选择一种方法而不是另一种方法?
以下两个查询的SQL生成方式有何不同?
方法一
通过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();
方法二
查找所有AuthoredItems包含当前项目ID的作者。
List<Authors> Authors = db.Authors
.Where(i => i.AuthoredItems.Any(a => a.ID == CurrentItemID))
.ToList();
像这样的多对多关系:
public class Item
{
...
ICollection<Author> Authors {get;set;}
}
public class Author
{
...
ICollection<Item> AuthoredItems {get;set;}
}
UPDATE
根据建议,我尝试为这些查询打印生成的sql。
方法1给我:
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])
)
第二种方法中的sql在我的脑海中容易得多。 而且联接的数量也更少了。我对吗,第二种方法会更有效吗?
第一个查询似乎更“自然”,更容易理解,因为您知道商品ID,并且一旦找到商品,就选择所有列出的作者。
在另一个查询中,您查找每个作者是否是项目的作者,如果是,则选择他。
我会选择第一个。 我不确定两个查询的速度差异,但是第一个查询肯定更容易理解。
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