将sql查询转换为linq但没有得到任何结果答案

【问题标题】：Converted sql query to linq but not getting any result将sql查询转换为linq但没有得到任何结果
【发布时间】：2017-11-08 10:41:59
【问题描述】：

因为我是 Linq 和实体框架的新手。

以下是我的工作存储过程，其中包含一个逗号分隔字符串的值（SUBSTRING 子句）

SELECT DISTINCT 
    SR.StudentRequestId, 
    SR.RegistrationId, 
    SR.Location,
    SR.PaymentMethod, 
    SR.CreatedOn, 
    C.ClassName, 
    CC.CampusName, 
    CASE WHEN ISNULL(TSR.StatusId,0)=0 THEN 1 ELSE TSR.StatusId END AS StatusId, 
    SUBSTRING(
        (SELECT', ' + REPLACE(REPLACE(ST1.FromTime,'AM',''),'PM','') + '-'+ ST1.ToTime AS [text()]
         FROM dbo.StudentRequestTimings ST1
         WHERE ST1.StudentRequestId = SRT.StudentRequestId
         ORDER BY ST1.CreatedOn FOR XML PATH ('')
        ), 2, 1000) [Time] FROM StudentRequest SR
    INNER JOIN Registration R ON R.RegistrationId = SR.RegistrationId
    INNER JOIN Campus CC ON CC.CampusId = R.CampusId
    INNER JOIN Class C ON C.ClassId = SR.ClassId
    LEFT JOIN TutorClasses TC ON SR.ClassId = TC.ClassId
    LEFT JOIN StudentRequestTimings SRT ON SR.StudentRequestId = SRT.StudentRequestId
    LEFT JOIN TutorStudentRequest TSR ON TSR.StudentRequestId = SRT.StudentRequestId AND TutorId = @RegistrationId
    WHERE TC.RegistrationId = @RegistrationId
    ORDER BY SR.CreatedOn DESC

我需要在接受IQueryable<T> 的PagedList 方法中使用此数据，并且我想将此SP 转换为返回IQueryable 的LINQ 查询（内部PagedList 方法使用@987654327 @ 和 .Take() 执行服务器端分页）。

以下是我迄今为止的尝试，但我不知道为什么我没有从 SQL 查询中获得预期的结果。我认为下面的代码有问题。任何人都可以找出以下代码中的问题吗？

var model = (from sr in db.StudentRequests
            join r in db.Registrations on sr.RegistrationId equals r.RegistrationId
           join cc in db.Campus on r.CampusId equals cc.CampusId
           join c in db.Classes on sr.ClassId equals c.ClassId
           from tc in db.TutorClasses.Where(t => t.ClassId == sr.ClassId).DefaultIfEmpty()
           from srt in db.StudentRequestTimings.Where(s => s.StudentRequestId == sr.StudentRequestId).DefaultIfEmpty()
           from tsr in db.TutorStudentRequests.Where(t => t.StudentRequestId == srt.StudentRequestId && t.TutorId == registrationid).DefaultIfEmpty()
           where tc.RegistrationId == registrationid
            select new TutorDashboard
            {
             StudentRequestId = sr.StudentRequestId,
             RegistrationId = sr.RegistrationId,
             Location = sr.Location,
             PaymentMethod = sr.PaymentMethod,
             CreatedOn = sr.CreatedOn,
             ClassName = c.ClassName,
             CampusName = cc.CampusName,
             Time = string.Join(",", db.StudentRequestTimings.Where(p => p.StudentRequestId == sr.StudentRequestId).Select(p => p.FromTime.ToString().Replace("AM", "").Replace("PM", "") + "-" + p.ToTime.ToString())),
          }).Distinct();

谁能帮我解释一下为什么我没有从上面的 linq 查询中得到正确的结果？

问题是在向 Linq 添加以下代码时我遇到了问题，我该如何解决这个问题

Time = string.Join(",", db.StudentRequestTimings.Where(p => p.StudentRequestId == sr.StudentRequestId).Select(p => p.FromTime.ToString().Replace("AM", "").Replace("PM", "") + "-" + p.ToTime.ToString())),

我添加了这段代码，因为我想从多行中获取单个字符串

1:00-1:30 PM, 2:00=2:30 PM
5:00-5:30 PM

同样明智

【问题讨论】：

LINQ 不能替代 SQL。它是 ORM 的一种语言。您的实体应该有关系和映射，而不是连接。除了db.StudentRequests.Where(sr=>sr.RegistrationID == registrationId).Select(...)，你不应该写任何东西
另一方面，您不需要 LINQ 对查询结果进行分页。在数据库中创建视图或函数，将 that 映射到 TutorDashboard 实体并使用 LINQ 查询该实体
我想我会更好地检查你 sql Profile 然后获取 Generated T-Sql 并将其复制并粘贴到 Management Studio 然后运行它，检查条件并加入以知道为什么不'不返回任何值
@PanagiotisKanavos 实际上我想在视图侧返回模型，所以我将它与 TutorDashboard 绑定
@Xtremcool 不会改变任何东西。将查询转换为视图（没有 distinct/order by），为其创建实体并将实体映射到视图。类似的问题还有很多

标签： c# sql-server entity-framework linq

【解决方案1】：

由于在转换逗号分隔代码时出现问题，我在下面的代码中实现了

var query = (from sr in db.StudentRequests
             join r in db.Registrations on sr.RegistrationId equals r.RegistrationId
             join cc in db.Campus on r.CampusId equals cc.CampusId
             join c in db.Classes on sr.ClassId equals c.ClassId
             from tc in db.TutorClasses.Where(t => t.ClassId == sr.ClassId).DefaultIfEmpty()
             from srt in db.StudentRequestTimings.Where(s => s.StudentRequestId == sr.StudentRequestId).DefaultIfEmpty()
             from tsr in db.TutorStudentRequests.Where(t => t.StudentRequestId == srt.StudentRequestId && t.TutorId == registrationid).DefaultIfEmpty()
             where tc.RegistrationId == registrationid
             select new
             {
               StudentRequestId = sr.StudentRequestId,
               RegistrationId = sr.RegistrationId,
               Location = sr.Location,
               PaymentMethod = sr.PaymentMethod,
               CreatedOn = sr.CreatedOn,
               ClassName = c.ClassName,
               CampusName = cc.CampusName,
               StatusId = tsr.StatusId == null ? 1 : tsr.StatusId,
               Time = db.StudentRequestTimings.Where(p => p.StudentRequestId == sr.StudentRequestId).Select(p => p.FromTime.ToString().Replace("AM", "").Replace("PM", "") + "-" + p.ToTime)
               }).ToList().GroupBy(p => new { p.StudentRequestId }).Select(g => g.First()).ToList();


          var model = query.AsEnumerable().Select(x => new TutorDashboard
           {
              StudentRequestId = x.StudentRequestId,
              RegistrationId = x.RegistrationId,
              Location = x.Location,
              PaymentMethod = x.PaymentMethod,
              CreatedOn = x.CreatedOn,
              ClassName = x.ClassName,
              CampusName = x.CampusName,
              StatusId = x.StatusId == null ? 1 : x.StatusId,
              Time = string.Join(",", x.Time),
               }).ToList().ToPagedList(page ?? 1, 1);

希望这段代码对一些需要逗号分隔值和多个左连接的人有所帮助

【讨论】：