c# - 实体框架中的空值

Question

var query = from section in load_sections.Sections
            join course in load_sections.Courses
                on section.Course_Id equals course.Course_Id
            join faculty in load_sections.Faculties
                on section.Faculty_Id equals faculty.Faculty_Id
            select section;

我的 section.Faculty_Id 中有一些空值，它们不等于 Faculty.Faculty_Id 中的任何行，它只是返回 section.Faculty_Id 不为空的记录...如果 section.Faculty_Id 不为空，那么它必须返回 Table Courses 的其他剩余字段

Answer 1

如果faculty由于某种原因无法删除连接，则必须构造一个外部连接：

var query = from section in load_sections.Sections
            join course in load_sections.Courses
                on section.Course_Id equals course.Course_Id
            join faculty in load_sections.Faculties
                on section.Faculty_Id equals faculty.Faculty_Id into faculties
            from f in faculties.DefaultIfEmpty()
            select section;

这执行一个GroupJoinwith Faculties。随后的效果from f in faculties是分组再次被 a 展平SelectMany。.DefaultIfEmpty()创建外连接。