Allowing duplicate rows is the only practical decision for the following reasons: 1. Most of the ways you can mess up in SQL involve failing to respect unique

Allowing duplicate rows is the only practical decision for the following reasons: 1. Most of the ways you can mess up in SQL involve failing to respect unique keys in joins, and this can be done just as easily whether or not duplicate rows are allowed. 2. Enforcing no duplicates would be very expensive.