C#在LINQ中左联接 [英] C# Left join in LINQ

问题描述

不确定如何将以下sql转换为LINQ表达式.我的数据库确实使用了引用完整性，并且表Content与表Content_Training的关系是一对多的(例如:1个内容可以具有许多content_trainings).

Not sure how to convert the following sql into a LINQ expression. My db does use referential integrity and table Content is related to table Content_Training in a 1 to many relationship (ex: 1 content can have many content_trainings).

select c.ContentId, c.Name, ct.TrainingTypeId 
from dbo.Content c left join dbo.ContentTraining ct on c.ContentId = ct.ContentId
where c.ExpirationDate is not null
order by ct.TrainingTypeId, c.Name

我已经尝试过了，这似乎行得通.但是，我不确定"let"关键字的用法.

I have tried this, which seems to work. However, I am not certain about the usage of the "let" keyword.

var data = (from c in context.Contents
let ct = ( from t in context.Content_Training where t.ContentId == c.ContentId
select new { t.TrainingTypeId } ).FirstOrDefault()
where c.ExpirationDate.HasValue
orderby ct.TrainingTypeId, c.Name
select new { c.ContentId, c.Name, ct.TrainingTypeId } ).ToList();

推荐答案

对于左联接，您需要使用DefaultIfEmpty()

For left join, you need to use DefaultIfEmpty()

您的查询应与此类似:

var query = from c in Content
            join ct in ContentTraining
            on c.ContentId equals ct.ContentId into g
            from ct in g.DefaultIfEmpty()
            where c.ExpirationDate != null
            select new
            {     
                c.ContentId, 
                c.Name, 
                ct.TrainingTypeId 
             }).ToList();

请参考 linq中的左外部联接

和

http://msdn.microsoft.com/en-us/library/bb397895.aspx

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