c# - .NET Linq 左连接

Question

我在 SQL 中有 2 个表:

Table 1 
Step Id
Step Name

Table 2
Profile Id
Step Id
Completed

即使表 2 中没有匹配项，我也想返回以下结果:

Results
Table1.Step Id
Table1.Step Name
Table2.Profile Id
Table2.Completed 

我在 SQL 中执行此操作的方式如下:

select * from [Table 1] t1
left join [Table 2] t2
on t1.Step Id = t2.Step Id

这会产生我期望的结果。

当我将其翻译成 linq 时:

public static List<UserCompletion> GetStepCompletion(string category, string profileid) { 

List<Step> step = GetSteps(category);
List<UserStep> userStep = GetUserSteps(category, profileId);    

var q = from s in step
         join us in userStep
         on s.Id equals us.StepId
         select new UserCompletion
         {
           StepId = s.Id,
           Headline = s.StepName,
           ProfileId = us.ProfileId
           Completed= us.Completed
          };

     return q.ToList();

}

它可以工作，但像 JOIN 而不是 left join。我只得到匹配的结果。

此外，UserCompletion 是我从此方法返回的一个对象。

Answer 1

您也可以试试这个(假设 us.Completed 是 bool 值):

var q = from s in step
         let us = (from i in userStep where s.Id = i.StepId).SingleOrDefault()
         select new UserCompletion
         {
           StepId = s.Id,
           Headline = s.StepName,
           ProfileId = us.ProfileId
           Completed = us == null ? false : us.Completed
          };

这不会变成 sql 中的连接，而是像这样的嵌套 select 语句:

select 
    StepId, Headline, ProfileId,
    isnull((select top(1) Completed from userStep where StepId = Id), 0) as Completed
from step