这样的双where的语句应该怎么写呢：
var test=MyList.Where(a => a.Flows.Where(b => b.CurrentUser == “”)
下面我就说说这个问题，想想有几种方法。先来做一下准备工作，我们使用最简单的模型Category和Post
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public class Category
{ public int Id { get; set; }
public string Name { get; set; }
public virtual ICollection Posts { get; set; }
}public class Post
{ public int Id { get; set; }
public string Author { get; set; }
public string Title { get; set; }
public int CategoryId { get; set; }
public Category Category { get; set; }
} |
把上面的问题转换成这个模型解释就是：查询含有某个Author写的Post的Category。
这个问题如果直接使用SQL来写的话很简单：
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select distinct(c.Id),c.Name from Categories c
inner join Posts p on p.CategoryId=c.Id
where p.Author=N'cj'
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下面依次来看EntityFramework的四种实现方法。
第一种，使用Any
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var list = ctx.Categories.Where(t => t.Posts.Any(s => s.Author == "cj"));
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生成的SQL语句如下：
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SELECT [Extent1].[Id] AS [Id],
[Extent1].[Name] AS [Name]
FROM [dbo].[Categories] AS [Extent1]
WHERE EXISTS (SELECT
1 AS [C1]
FROM [dbo].[Posts] AS [Extent2]
WHERE ([Extent1].[Id] = [Extent2].[CategoryId]) AND (N'cj' = [Extent2].[Author])
)
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第二种，使用Select
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var list = ctx.Posts.Where(t => t.Author == "cj").Select(t => t.Category).Distinct();
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生成的SQL语句如下：
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SELECT [Distinct1].[Id] AS [Id],
[Distinct1].[Name] AS [Name]
FROM ( SELECT DISTINCT
[Extent2].[Id] AS [Id],
[Extent2].[Name] AS [Name]
FROM [dbo].[Posts] AS [Extent1]
INNER JOIN [dbo].[Categories] AS [Extent2] ON [Extent1].[CategoryId] = [Extent2].[Id]
WHERE N'cj' = [Extent1].[Author]
) AS [Distinct1]
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第三种，使用SelectMany
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var list = ctx.Categories.SelectMany(t => t.Posts, (category, post) => new
{ category,
post
}).Where(t => t.post.Author == "cj").Select(t => t.category).Distinct();
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生成的SQL语句如下：
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SELECT [Distinct1].[Id] AS [Id],
[Distinct1].[Name] AS [Name]
FROM ( SELECT DISTINCT
[Extent1].[Id] AS [Id],
[Extent1].[Name] AS [Name]
FROM [dbo].[Categories] AS [Extent1]
INNER JOIN [dbo].[Posts] AS [Extent2] ON [Extent1].[Id] = [Extent2].[CategoryId]
WHERE N'cj' = [Extent2].[Author]
) AS [Distinct1]
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第四种，还是使用SelectMany
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var list = ctx.Categories.SelectMany(t => t.Posts).Where(t => t.Author == "cj").Select(t => t.Category).Distinct();
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生成的SQL语句如下：
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SELECT [Distinct1].[Id] AS [Id],
[Distinct1].[Name] AS [Name]
FROM ( SELECT DISTINCT
[Extent1].[Id] AS [Id],
[Extent1].[Name] AS [Name]
FROM [dbo].[Categories] AS [Extent1]
INNER JOIN [dbo].[Posts] AS [Extent2] ON ([Extent1].[Id] = [Extent2].[CategoryId]) AND ([Extent2].[CategoryId] = [Extent1].[Id])
WHERE N'cj' = [Extent2].[Author]
) AS [Distinct1]
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下面分别来说说这四种方法：
第一种方法Any，更符合我们的查询习惯，也就是文章开始提到的问题的查询风格，只不过里面的Where应该换成Any
第二种方法Select，生成的SQL语句，跟我们自己写的SQL语句是一样的，这种方法以Post为查询主体，好处可以看看SQL语句优化方面的知识。
第三种和第四种都是SelectMany，虽然EF的查询写法不同，但生成的SQL语句完全相同， 当然SelectMany是以Category为查询主体。关于SelectMany的用法请参考MSDN。
补充（2014-5-11）
第五种方法，使用Contains
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var list = ctx.Categories.Where(t => t.Posts.Select(s => s.Author).Contains("cj"));
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生成的SQL语句如下：
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SELECT [Extent1].[Id] AS [Id],
[Extent1].[Name] AS [Name]
FROM [dbo].[Categories] AS [Extent1]
WHERE EXISTS (SELECT
1 AS [C1]
FROM [dbo].[Posts] AS [Extent2]
WHERE ([Extent1].[Id] = [Extent2].[CategoryId]) AND (N'cj' = [Extent2].[Author])
)
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这种方法和第一种方法Any生成的SQL语句是一样一样的。