I've had lots of problems with surrogate primary keys where people don't put enough constraints on the natural key and then duplicates (sometimes slightly diffe

I've had lots of problems with surrogate primary keys where people don't put enough constraints on the natural key and then duplicates (sometimes slightly different) get in. Unless you're using innodb, primary key is not special, it's just another index. If you decide to use surrogate keys you also need to enforce the natural key too (if possible). There are also some nice performance advantages you can get from natural keys with covering composite indices in the case where you only want the key. But in general a table can have multiple keys in all but the highest levels of normalisation.