EF架构~linq模拟left join的两种写法,性能差之千里!

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对于SQL左外连接我想没什么可说的,left join将左表数据都获出来,右表数据如果在左表中不存在,结果为NULL,而对于LINQ来说,要实现left join的效果,也是可以的,在进行join时直接into到集合变量就可以了,但在赋值时,如果只需要集合的一条记录,那在写法上又会有两种,而这两种写法所产生的性能是相关千里的,下面来看一下.

首先是SQL的左外连接

SELECT  [t6].[CourseID] ,
        [t6].[UserID] ,
        [t6].[CourseName] ,
        [t6].[ResourceID] ,
        [t6].[StudyTime] ,
        [t6].[BeginTime] ,
        [t6].[EndTime] ,
        [t6].[value] AS [CategoryID] ,
        [t6].[value2] AS [ClassHour] ,
        [t6].[value3] AS [Percent] ,
        [t6].[test] ,
        [t6].[ID] ,
        [t6].[SmallPicture]
FROM    dbo.User_Course AS t6
        LEFT  OUTER JOIN [User_StudyRecord] AS t3 ON t6.UserID = t3.UserID
                                                     AND t3.ResourceID = t6.ResourceID

当它被翻译成LINQ之后,是分页产生的结果,所以感觉更很乱了,呵呵,(LINQ在翻译SQL时,本来就够乱的,再一分页,用上row_number,更乱了),但结果是一样的,

咱们就不管微软是怎么翻译的了

我们重要是看一下,实现时LINQ代码的写法

第一种,性能低下的,如果结果为20条记录,那它需要多连接20次

            var linq = from _data in new User_Course(UnitOfWork).GetEntities()
                       let list = new Res_Item(UnitOfWork).GetEntities().Select(t => new Entity.Res_Item
                       {
                           ID = t.ID,
                           SmallPicture = t.SmallPicture,
                       }).Where(i => i.ID == _data.ResourceID)
           
                       let list2 = new User_StudyRecord(UnitOfWork).GetModel().Select(r => new Entity.User_StudyRecord
                       {
                           StudyContent = r.StudyContent,
                           UserID = r.UserID,
                           Res_ItemID = r.Res_ItemID,
                       }).Where(i => i.Res_ItemID == _data.ResourceID && i.UserID == _data.UserID)
                       from record in list2.DefaultIfEmpty()
                       where _data.CategoryID != (int)CustomEnum.CategoryType.BroadcastProgram
                       && _data.CategoryID != (int)CustomEnum.CategoryType.AskRoom
                       select new Entity.User_Course
                       {
                           CourseID = _data.CourseID,
                           UserID = _data.UserID,
                           CourseName = _data.CourseName,
                           ResourceID = _data.ResourceID,
                           StudyTime = _data.StudyTime,
                           BeginTime = _data.BeginTime,
                           EndTime = _data.EndTime,
                           CategoryID = _data.CategoryID ?? 2,
                           Res_Item = list.FirstOrDefault(),
                           ClassHour = _data.ClassHour ?? 0,
                           Percent = _data.Percent ?? 0,
                           Prev_ResourceName = record == null ? string.Empty : record.StudyContent,
                       };