The problem with SQL is it is based on relational algebra, just not consistently. SQL allows you to insert a duplicate row or update a row such that it becomes

The problem with SQL is it is based on relational algebra, just not consistently. SQL allows you to insert a duplicate row or update a row such that it becomes similar to another row. But then it is not possible to delete or change the row again without changing or deleting all the duplicates, because the query language assumes rows are unique. This is obviously a footgun and not useful behavior.