相关疑难解决方法(0)

有人可以帮我解决这个例外吗？我不明白它意味着什么或如何解决它...这是一个SqlException,带有以下消息:

使用UNION,INTERSECT或EXCEPT运算符组合的所有查询在其目标列表中必须具有相同数量的表达式.

我在伪代码中运行查询时得到它,如下所示:

// Some filtering of data
var query = data.Subjects
            .Where(has value)
            .Where(has other value among some set of values);

// More filtering, where I need to have two different options
var a = query
            .Where(some foreign key is null);
var b = query
            .Where(some foreign key is not null)
            .Where(and that foreign key has a property which is what I want);
query = a.Union(b);

// Final filter and then get result as a list
var list = …