Linq多表左外连接left join查询

左外连接会检索出LEFT JOIN左表中的所有行，而不管右表是否有匹配项。

今天的一个项目中，需要将classlist表与其他3张表进行连接查询，且classlist表中某列为空的数据也需要查询出来，这就需要进行LEFT JOIN（左外连接查询），且项目使用的是Entity Framework框架，因此，可以使用Linq语句进行左外连接查询。

通过左连接4张表（klc_classlist、klc_object 、klc_student 、klc_license ）进行查询，语句如下：

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// linq语句多表联合查询 var classlist = from a in dc.klc_classlist                 join b in dc.klc_object on a.object_id equals b.id into result1                 from ab in result1.DefaultIfEmpty()                 join c in dc.klc_student on a.student_id equals c.id into result2                 from abc in result2.DefaultIfEmpty()                 join d in dc.klc_license on abc.license_id equals d.id into result3                 from abcd in result3.DefaultIfEmpty()                 where a.order_list.Contains(stu_id_str) && (a.student_id == stu_id || a.student_id == null)                 select new                 {                     id = a.id,                     date = a.date,                     start_time = a.start_time,                     end_time = a.end_time,                     duration = a.duration,                     object_name = ab.name,                     student_name = abc.name,                     status = a.status                 };