今天在群里提到下面的问题

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

    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：

select distinct(c.Id),c.Name from Categories c
inner join Posts p on p.CategoryId=c.Id
where p.Author=N'cj'

下面依次来看EntityFramework的四种实现方法。

第一种，使用Any

                var list = ctx.Categories.Where(t => t.Posts.Any(s => s.Author == "cj"));

生成的SQL语句如下：

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])
    )

 第二种，使用Select

                var list = ctx.Posts.Where(t => t.Author == "cj").Select(t => t.Category).Distinct();

生成的SQL语句如下：

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]

 第三种，使用SelectMany

                var list = ctx.Categories.SelectMany(t => t.Posts, (category, post) => new
                {
                    category,
                    post
                }).Where(t => t.post.Author == "cj").Select(t => t.category).Distinct();

生成的SQL语句如下：

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]

 第四种，还是使用SelectMany

                var list = ctx.Categories.SelectMany(t => t.Posts).Where(t => t.Author == "cj").Select(t => t.Category).Distinct();

生成的SQL语句如下：

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]

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。

 Demo下载地址

补充（2014-5-11）

第五种方法，使用Contains

                var list = ctx.Categories.Where(t => t.Posts.Select(s => s.Author).Contains("cj"));

生成的SQL语句如下：

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])
    )

这种方法和第一种方法Any生成的SQL语句是一样一样的。