如何在LINQ to Entities中进行LEFT JOIN？

Question

我花了最近两天试图找出如何在LINQ中进行真正的LEFT JOIN并且我没有成功.我有一个用户表,其中有一个"Primary2Address"列,可能并且通常为NULL.所以我必须在这里做LEFT JOIN.另外,在Address表中,我有更多可能为NULL的关系,所以我必须做多个LEFT JOINS.
每次LINQ尝试我都会用UNIONS输出一些严重的CRAZY sql语句,嵌套的SELECT语句和更古怪的东西.
我只需要:

SELECT u.UserName FROM Users u 
LEFT JOIN Addresses a ON a.AddressiD = u.Primary2Address
LEFT JOIN States s ON s.StateID = a.Address2State
LEFT JOIN Countries c ON c.CountryID = a.CountryID

请帮忙!到目前为止,我的解决方法是创建一个使用上面的sql语句的存储过程,但我真的想尝试使用LINQ(L2E).多谢你们!

Answer 1

DefaultIfEmpty 用于EntityFramework 4+的左连接

var query = from u in context.Users
            from a in context.Addresses
                             .Where(x => u.Primary2Address == x.AddressiD)
                             .DefaultIfEmpty()
            from s in context.States
                             .Where(x => a.Address2State == x.StateID)
                             .DefaultIfEmpty()
            from c in context.Countries
                             .Where(x => a.CountryID == x.CountryID)
                             .DefaultIfEmpty()
            select u.UserName;