.NET Linq加入

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,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

这产生了我期望的结果.

当我把它翻译成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而不是左连接.我只收到匹配的结果.

此外,UserCompletion是我从此方法返回的对象.

我已经在这里敲了几天......任何帮助都将不胜感激.

Answer 1

你也可以尝试这个(假设us.Completed是布尔值):

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,而是嵌套的select语句,如下所示:

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