SQL is not relational algebra. I think the more mathematic ideas in the article should be viewed in the context of relational alebra and not in the context of SQL.
So, the problems with NULLs is an SQL problem not a relational algebra problem. The 'flip side' is really the flip side of relational algebra and not SQL even though the article is not very clear in keeping the two separate.
SQL is not relational algebra but rather an attempt to bridge a relational algebra/programming gap. The null issue arises in SQL due to it being used in a relational algebra context in one case and a software development context in another. The point though is that these are treated as equivalents because of an open world assumption.
Comments
SQL is not relational algebra. I think the more mathematic ideas in the article should be viewed in the context of relational alebra and not in the context of SQL.
So, the problems with NULLs is an SQL problem not a relational algebra problem. The 'flip side' is really the flip side of relational algebra and not SQL even though the article is not very clear in keeping the two separate.
SQL is not relational algebra but rather an attempt to bridge a relational algebra/programming gap. The null issue arises in SQL due to it being used in a relational algebra context in one case and a software development context in another. The point though is that these are treated as equivalents because of an open world assumption.