In most actual implementations of relational DBs there's nothing to disallow repetition in a table - you just don't specify a primary key (or preferably have a

In most actual implementations of relational DBs there's nothing to disallow repetition in a table - you just don't specify a primary key (or preferably have a unique primary key that's just an ignored surrogate). The difficulty with caring about ordering in the relational model is that suddenly you're imposing a lot more constraints: when you're joining two tables, whose order 'wins'? If you say (for example) that left side wins, you then need to keep track of that order through your query.