如何在sql中使用的linq中使用Left join? [英] How can I use Left join in linq that we use in sql?

问题描述

如何在编写SQL查询的Linq中使用左联接?

How can I use Left join in Linq that I write SQL query?

select 
    p.Name, p.Family,
    E.EmployTypecode, E.employtypeName, E.EmplytyppeTye 
from 
    personnel as p
left join 
    Employee as E on E.EmployTypecode = p.EmployTypecode 

推荐答案

使用Join关键字而不是Left join，并且必须使用"INTO"关键字和"DefaultIfEmpty()"方法，因为右表将返回空值.

Use Join keyword instead of Left join and it is mandatory to use "INTO" keyword and "DefaultIfEmpty()" method as right table returns null value.

   var query = from p in personnel 
               join e in Employee on p.EmployTypecode equals e.EmployTypecode into t
               from nt in t.DefaultIfEmpty()
               orderby p.Name

    select new
    {
        p.Name, p.Family,
        EmployTypecode=(int?)nt.EmployTypecode,  // To handle null value if Employtypecode is specified as not null in Employee table.
        nt.employtypeName, nt.EmplytyppeTye
    }.ToList();

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