EF 的 DefaultIfEmpty()

static void Main(string[] args)
        {
            using (var db = new BlogContext())
            {
                db.Database.Log = Console.WriteLine;

                var query = from a in db.Blogs
                            join b in db.Posts on a.BlogId equals b.BlogId into jjj
                            from t in jjj.DefaultIfEmpty()
                            select a;

                foreach (var blog in query)
                {
                    Console.WriteLine(blog.Name);
                }

                Console.WriteLine("-----------------------------------");
                var query2 = from a in db.Blogs
                            join b in db.Posts on a.BlogId equals b.BlogId into jjj
                            select a;


                foreach (var blog in query2)
                {
                    Console.WriteLine(blog.Name);
                }

            }

            Console.WriteLine("Press any key to exit...");
            Console.ReadKey();
        }

兩個表如下 

 

這兩個查詢生成的SQL 差別在哪兒？？？

SELECT
    [Extent1].[BlogId] AS [BlogId],
    [Extent1].[Name] AS [Name],
    [Extent1].[Url] AS [Url],
    [Extent1].[Rating] AS [Rating]
    FROM  [dbo].[Blogs] AS [Extent1]
    LEFT OUTER JOIN [dbo].[Posts] AS [Extent2] ON [Extent1].[BlogId] = [Extent2].[BlogId]

-- 正在 2019/3/12 10:44:02 +08:00
 執行
-- 已在 2 毫秒內完成，結果爲: SqlDataReader
Blog 1
Blog 2
Blog 2
Blog 3
已於 2019/3/12 10:44:02 +08:00
 關閉了連接
-----------------------------------
已於 2019/3/12 10:44:02 +08:00
 打開了連接
SELECT
    [Extent1].[BlogId] AS [BlogId],
    [Extent1].[Name] AS [Name],
    [Extent1].[Url] AS [Url],
    [Extent1].[Rating] AS [Rating]
    FROM [dbo].[Blogs] AS [Extent1]


-- 正在 2019/3/12 10:44:02 +08:00
 執行
-- 已在 0 毫秒內完成，結果爲: SqlDataReader

Blog 1
Blog 2
Blog 3

第二個查詢的join 沒用！

*************************************************************************************************************************************************鏈接查詢也是先要做交叉查詢。

/****** Script for SelectTopNRows command from SSMS  ******/
SELECT p.Title, [Name]
  FROM [MigrationsDemo.Model+BlogContext].[dbo].[Blogs] m left join  [MigrationsDemo.Model+BlogContext].[dbo].[Posts] p on 1=1


SELECT p.Title,  [Name]
  FROM [MigrationsDemo.Model+BlogContext].[dbo].[Blogs] m inner join  [MigrationsDemo.Model+BlogContext].[dbo].[Posts] p on 1=1


SELECT p.Title,  [Name]
  FROM [MigrationsDemo.Model+BlogContext].[dbo].[Blogs] m, [MigrationsDemo.Model+BlogContext].[dbo].[Posts] p 

這3個查詢的結果一樣，執行計劃也一樣？