linq-to-sql - LINQ to SQL - where 子句中的可为空类型

Question

我有一个包含空值列的表......当我尝试查询该列为空的记录时:

这有效:

        var list = from mt in db.MY_TABLE
                   where mt.PARENT_KEY == null
                   select new { mt.NAME };

这不会:

        int? id = null;
        var list = from mt in db.MY_TABLE
                   where mt.PARENT_KEY == id
                   select new { mt.NAME };

为什么？

Answer 1

经过更多的谷歌搜索，我找到了答案:

ref #1

ref #2

int? id = null;
var list = from mt in db.MY_TABLE
           where object.Equals(mt.PARENT_KEY, id)  //use object.Equals for nullable field
           select new { mt.NAME };

此 LINQ 按如下方式呈现给 SQL:

((mt.PARENT_KEY IS NULL) AND (@id IS NULL)) 
OR ((mt.PARENT_KEY IS NOT NULL) AND (@id IS NOT NULL) AND (mt.PARENT_KEY = @id))