c# – 关于UNION,INTERSECT和EXCEPT的SqlException

All queries combined using a UNION,INTERSECT or EXCEPT operator must have an equal number of expressions in their target lists.

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 = query
            .Where(last requirement)
            .ToList();

如果我删除了a.Union(b)部分,它会毫无例外地运行.所以我知道错误就在那里.但为什么我会得到它？我该如何解决？我在这里做的事太疯狂吗？我误解了如何使用联盟的东西吗？

基本上我所拥有的是一些具有其他实体的外键的实体.我需要将所有将该外键设置为null的实体或该外来实体满足某些要求的实体.