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

如果您出于某种原因无法删除faculty 上的联接，则必须构造一个外部联接：

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;

这将执行GroupJoin 和Faculties。后面的from f in faculties的效果是分组又被SelectMany拉平了。 .DefaultIfEmpty() 创建外连接。