SQL tables are multisets (sets allowing duplicates) rather than sets as defined by the relational model. This was likely a deliberate decision in order to make

SQL tables are multisets (sets allowing duplicates) rather than sets as defined by the relational model. This was likely a deliberate decision in order to make implementations simpler and faster, since it avoids some uniqueness checks. For example it is much simpler to implement UNION if you don't have to check for duplicates. But arguably this push some complexity to the user, since duplicates lead to all kinds of problems. On the other hand three-valued logic is obviously more complex than regular boolean logic. But I don't agree with the author that three-valued is in conflict with the relational model.