关系数据库中的空值是否正常？

There's a school of thought that null values should not be allowed in a relational database. That is, a table's attribute (column) should not allow null values. Coming from a software development background, I really don't understand this. It seems that if null is valid within the context of the attribute, then it should be allowed. This is very common in Java where object references are often null. Not having an extensive database experience, I wonder if I'm missing something here.There's a school of thought that null values sh