Lambda方式左連接有Linq方式左連接

網上查到的直接使用Join+DefaultIfEmpty的方式是錯誤的，實際生成SQL是兩表先內聯接，然后再LEFT JOIN。經過查證，參考資料，最終得到如下兩種方式的左連接寫法：

        public void TestMethod1()
        {
            MettingEntities db = new MettingEntities();
            
            //LINQ方式
            var query = (from st in db.met_SysItemsClass
                        join cl in db.met_Shortcut on st.ClassID equals cl.ClassID into cls
                        from c in cls.DefaultIfEmpty()
                        select new TestModel
                        {
                            ClassID = st.ClassID,
                            ClassName = st.ClassName,
                            AdminID = c.AdminID
                        }).ToList();

            //Lambda 方式
            var list = db.met_SysItemsClass.GroupJoin(db.met_Shortcut, x => x.ClassID, y => y.ClassID, (x, y) => y.DefaultIfEmpty().Select(z => new TestModel { ClassID = x.ClassID, ClassName = x.ClassName, AdminID = z.AdminID })).SelectMany(x => x).ToList();

        }
    }

    public class TestModel
    {
        public int? AdminID { get; set; }
        public int ClassID { get; set; }
        public string ClassName { get; set; }
    }

兩種方式生成的SQL語句完全相同，如下：

SELECT 
[Extent1].[ClassID] AS [ClassID], 
[Extent1].[ClassName] AS [ClassName], 
[Extent2].[AdminID] AS [AdminID]
FROM  [dbo].[met_SysItemsClass] AS [Extent1]
LEFT OUTER JOIN [dbo].[met_Shortcut] AS [Extent2] ON [Extent1].[ClassID] = [Extent2].[ClassID]